Short Notes on Alaska Geology 1999

Professional Report 119 State of Alaska Department of Natural Resources Division of Geological & Geophysical Surveys

Milton A. Wiltse State Geologist 1999 EDITORIAL POLICY Short Notes on Alaska Geology is a collection of brief scientific papers describing recent geological investigations of limited scope. This publication is widely distributed to a state, national, and international audience that represents industry, academia, government agencies, and the general public. Manuscripts are accepted for consideration with the understanding that they have not been previously published and are not being submitted for publication elsewhere, that all persons listed as authors have given their approval for submission of the paper, and that any person cited as a source of personal communication has approved such a citation. Authors are encouraged to suggest appropriate reviewers and provide reviewers’ telephone numbers.

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Division of Geological & Geophysical Surveys Professional Report 119 Recent research on Alaska geology Fairbanks, Alaska 2000

Front cover photo: Marine sedimentary rocks of the Nutzotin Mountains sequence (black strata in the foreground) in fault contact with the Wrangellia composite terrane in the Nutzotin Mountains, eastern Alaska Range. White strata in the midground is the Nizina Limestone. Gray strata in the background at the ridge crest is the Nikolai Greenstone. For scale, a person with a blue shirt can be seen on the ridgeline. View is to the north. (Photo by Jeffrey D. Manuszak) Back cover photo (top): View eastward along the west fork of Atigun River of prominent anticlinal hinge in the upper Kanayut Conglomerate (Lower Mississippian to Upper Devonian) within the Toyuk thrust zone, a major thrust zone in the central Brooks Range. See Chmielowski and others (this volume), “Duplex structure and Paleocene displacement of the Toyuk thrust zone near the Dalton Highway, north-central Brooks Range.” (Photo by Reia M. Chmielowski) Back cover photo (bottom): Excavator and bulldozer trench on Lewis Ridge, Donlin Creek property, southwestern Alaska. Trench is approximately 1,300 feet long and part of a 3.7-mile trenching program by Placer Dome Exploration Inc. during the 1997 and 1998 field seasons. Light-colored rocks are igneous dikes and sills, and darker rocks are Kuskokwim Group shale and graywacke. Note the core drill rig in the background to the right of center. (Photo by David J. Szumigala)

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OREWORD

The biennial Short Notes on Alaska Geology volume is a publication supported by Alaska’s entire geologic community. Alaska has a large and extended community of highly talented earth scientists, many of whom encounter geologic issues of great interest in the course of performing work associated with their primary obligations. In the press to meet organizational mandates, many of these stories could be lost in spite of the interest they hold for all of us. This realization led the Alaska Division of Geological & Geophysical Surveys to establish the Short Notes on Alaska STATE OF ALASKA Geology series so that there would be a vehicle to share Tony Knowles, Governor these insights. By intent and tradition the Short Notes on Alaska Geology series has been an inclusive publication. DEPARTMENT OF NATURAL RESOURCES Contributors to this biennial publication step forward from John T. Shively, Commissioner the ranks of private-sector firms, academia, and government agencies. DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS This volume of Short Notes on Alaska Geology 1999 is the Milton A. Wiltse culmination of two years of predominantly volunteer efforts State Geologist and Director by your colleagues to author 11 articles about stratigraphic, structural, geochronologic, mineral deposit, and paleontologic geologic phenomena observed within the state. These papers contain significant observations and new Division of Geological & Geophysical Surveys data with regional implications that advance the knowledge publications may be inspected at the following of Alaska’s geologic framework and will be cited frequently locations. Address mail orders to the Fairbanks in years to come. office. Editing the “Short Notes” volume, like authoring the papers Alaska Division of Geological & it contains, is an extra duty placed on top of ongoing work. Geophysical Surveys This year we owe our thanks to DeAnne Pinney, who Attn: Geologic Communications Section 794 University Avenue, Suite 200 graciously stepped up to that task. I believe that everyone Fairbanks, Alaska 99709-3645 who has participated in keeping the DGGS Short Notes on http://wwwdggs.dnr.state.ak.us Alaska Geology publication series viable over the years has [email protected] demonstrated a commitment to the finest traditions of Department of Natural Resources collegial science. Authors, editors, reviewers, and Public Information Center publication staff have labored diligently for our benefit. To 550 W 7th Ave., Suite 1250 all of them, I say thank you. Anchorage, Alaska 99501-3557

This publication, released by the Division of Geologi- cal & Geophysical Surveys, was produced and printed Milton A. Wiltse in Fairbanks, by Commerical Printing, at a cost of State Geologist and Director $9.00 per copy. Publication is required by Alaska Statute 41, “to determine the potential of Alaskan land for production of metals, minerals, fuels, and geother- mal resources; the location and supplies of groundwater and construction materials; the potential geologic haz- ards to buildings, roads, bridges, and other installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geology of Alaska.”

ii CONTENTS

The Soda Creek Limestone, a new upper Lower Devonian formation in the Medfra Quadrangle, west-central Alaska R.B. Blodgett, D.M. Rohr, E.A. Measures, N.M. Savage, A.E.H. Pedder, and R.W. Chalmers 1

Duplex structure and Paleocene displacement of the Toyuk thrust zone near the Dalton Highway, north-central Brooks Range R.M. Chmielowski, W.K. Wallace , and P.B. O’Sullivan 11

Borehole breakouts and implications for regional in situ stress patterns of the northeastern North Slope, Alaska C.L. Hanks, M. Parker, and E.B. Jemison 33

Measured section and interpretation of the Tingmerkpuk sandstone (Neocomian), northwestern DeLong Mountains, western Arctic Slope, Alaska D.L. LePain, K.E. Adams, and C.G. Mull 45

Stratigraphic architecture of the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence, Nutzotin and Mentasta mountains, Alaska J.D. Manuszak and K.D. Ridgway 63

Stratigraphy, depositional systems, and age of the Tertiary White Mountain basin, Denali fault system, southwestern Alaska K.D. Ridgway, J.M. Trop, and A.R. Sweet 77

Late Devonian (early Frasnian) conodonts from Denali National Park, Alaska N.M. Savage, R.B. Blodgett, and P.F. Brease 85

Geology and gold mineralization at the Donlin Creek prospects, southwestern Alaska D.J. Szumigala, S.P. Dodd, and A. Arribas, Jr. 91

Preliminary 40Ar/39Ar ages from two units in the Usibelli Group, Healy, Alaska: New light on some old problems D.M. Triplehorn, J. Drake, and P.W. Layer 117

Sedimentology and provenance of the Paleocene–Eocene Arkose Ridge Formation, Cook Inlet–Matanuska Valley forearc basin, southern Alaska J.M. Trop and K.D. Ridgway 129

Late Devonian (Late Famennian) radiolarians from the Chulitna Terrane, south-central Alaska M.Z. Won, R.B. Blodgett, K.H. Clautice, and R.J. Newberry 145

Previous Editions of Short Notes on Alaska Geology 153

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THE SODA CREEK LIMESTONE, A NEW UPPER LOWER DEVONIAN ORMATION IN THE MEDRA QUADRANGLE, WEST-CENTRAL ALASKA Robert B. Blodgett,1 David M. Rohr,2 Elizabeth A. Measures,2 Norman M. Savage,3 Alan E. H. Pedder,4 and Robert W. Chalmers2

ABSTRACT

The Soda Creek Limestone, a new stratigraphic unit of formation rank, is proposed here for a distinctive succession of richly fossiliferous, upper Lower Devonian (Emsian) limestone, shaly limestone, and minor shale exposed throughout much of the Medfra B-3 Quadrangle. The formation, bounded above and below by dolostone, is excellently exposed along a north-trending ridge in the eastern half of the SW¼ of sec. 23, T. 24 S., R. 23 E., Medfra B-3 Quadrangle. On this ridge, long known informally as “Reef Ridge” by mineral company geologists, the Emsian succession is repeated once by thrust faulting. The southernmost section on this ridge is designated as the type section, while the northern section is designated as the reference section. The 149.3-m-thick type section of limestone, shaly limestone, and minor shale represents shallow, subtidal to open-shelf depositional environments. Carbonate rocks consist of packstone, wackestone, and mudstone, and contain open-marine faunas consisting of brachiopods, rugose and tabulate corals, echinoderms, ostracodes, gastropods, and rare trilobites. Favositid tabulate corals up to 1 m in diameter are locally abundant. The lowermost beds of the unit are weakly dolomitized. Some channels of probable tidal origin with reworked bioclasts and intraclasts are present. Four prominent fossiliferous shale intervals occur interbedded with the carbonate rocks. Fossils from the new formation (notably brachiopods, corals, and conodonts) indicate an early Emsian age. This formation appears to have been deposited in an inner shelf setting of the platform carbonate complex that comprises the Nixon Fork subterrane of the Farewell terrane. Recent biogeographic studies of Devonian age faunas from the Farewell terrane, including the richly diverse fauna of the Soda Creek Limestone, indicate that this terrane probably originated as a rift block derived from the Siberian (or Angaraland) paleocontinent.

INTRODUCTION

The northern Kuskokwim Mountains of the Medfra both papers was on faunas from other regions. The Quadrangle, west-central Alaska, contains a richly Paleozoic stratigraphic succession of the northern fossiliferous succession of early to middle Paleozoic age Kuskokwim Mountains served as the type area for the strata, mostly deposited on a carbonate platform. Although definition of the Nixon Fork terrane of Patton (1978), these rocks are well exposed, very few investigations have which is characterized primarily by a distinctive been carried out upon them until recently. The basic succession of lower and middle Paleozoic platform stratigraphic framework for the Ordovician to Devonian carbonate rocks. Subsequently, the Nixon Fork terrane succession was established in the formal naming paper of (fig. 1) was reduced in rank by Decker and others (1994) Dutro and Patton (1982), and a generalized geologic map to that of a subterrane of the newly established Farewell was published by Patton and others (1980). Since this terrane, a larger tectonostratigraphic entity, also including pioneering stratigraphic work, numerous paleontologic the genetically related Dillinger and Mystic subterranes. and stratigraphic papers have appeared which have These subterranes are now commonly believed to be in addressed various parts of the Paleozoic succession in close stratigraphic juxtaposition (that is, facies relations) further detail (Blodgett and Rohr, 1989; Blodgett and with one another. Recent work on the biogeographic others, 1988; Frýda and Blodgett, 1998; Hahn and Hahn, affinities of Devonian age faunas of the Farewell terrane 1993; Measures and others, 1992a, b; Rohr, 1993; Rohr indicate that it probably originated as a rift block from the and Blodgett, 1988; Rohr and others, 1991, 1992; Rohr Siberian (or Angaraland) paleocontinent (Blodgett, 1998; and Gubanov, 1997; Savage and others, 1995; Stock, Blodgett and Boucot, 1999; Blodgett and Brease, 1997; 1981). In addition, Blodgett (1992) and Blodgett and Blodgett and others, 1999). Johnson (1992) described several Devonian gastropods In this paper, we define a new formation of late Early that occur in the Medfra Quadrangle, though the focus of Devonian (early Emsian) age which forms a distinctive

1Departments of Zoology and Geosciences, Oregon State University, Corvallis, Oregon 97331. Email for Robert Blodgett: [email protected] 2Department of Geology, Sul Ross State University, Alpine, Texas 79832. 3Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403. 48859 Park Pacific Terrace, Sidney, British Columbia V8L 4S1, Canada.

1 2SHORT NOTES ON ALASKA GEOLOGY 1999 BLODGETT AND OTHERS

Figure 1. Index map of part of west-central and southwestern Alaska showing location of Nixon Fork subterrane (stippled areas) of the Farewell terrane and the location of the “Reef Ridge” area in the Medfra B-3 Quadrangle; modified from Blodgett and Gilbert (1992) and Babcock and others (1995). marker unit of limestone and minor shale in the Medfra previously on the biostratigraphic and depositional B-3 Quadrangle of west-central Alaska. This unit is part environments, respectively, of this distinctive new of the previously described Whirlwind Creek Formation formation named herein the Soda Creek Limestone. The of Dutro and Patton (1982), which they described as “an Whirlwind Creek Formation of Dutro and Patton (1982) Upper Silurian to Upper Devonian sequence of includes several distinctive stratigraphic units, and is predominantly shallow-water carbonate rocks, 1,000– considered here to represent a stratigraphic unit of Group 1,500 m thick” (p. H19). The type section was designated rank. Its uppermost strata as recognized by the authors as a south-dipping sequence exposed on the ridge are stratigraphically equivalent to the Cheeneetnuk between Whirlwind and Soda creeks in sec. 23, T. 24 S., Limestone of Blodgett and Gilbert (1983) of the R. 23 E., Medfra B-3 Quadrangle. An additional McGrath A-4 and A-5 quadrangles, west-central Alaska. supplemental section was designated for the higher beds The upper part (early Eifelian age) of the Cheeneetnuk of the formation on the north flank of the syncline in secs. Limestone contains the same fauna and similar lithology 29 and 30, T. 23 S., R. 25 E., Medfra B-3 Quadrangle. as that found in the uppermost strata of the Whirlwind The Soda Creek Limestone (here named) corresponds to Creek Formation. The uppermost part of the Whirlwind the Emsian–Siegenian age interval of limestones with Creek was indicated by Dutro and Patton (1982) to be of coral biostromes shown in figure 5 of Dutro and Patton early Late Devonian (Frasnian) age. Examination by (1982). Although not mentioned in their naming paper, R.B. Blodgett of the site indicated to him by W.W. this distinctive unit is repeated once along their Patton, Jr. (USGS locality 9755-SD; field locality designated type section ridge of the Whirlwind Creek 76APa65c of W.W. Patton, Jr.) to be of Frasnian age, Formation. Several abstracts (Blodgett and others, 1995; shows that these strata belong to the herein-named Soda Chalmers and others, 1995) have been published Creek Limestone of Emsian age. No rugose corals SODA CREEK LIMESTONE, A NEW UPPER LOWER DEVONIAN ORMATION IN THE MEDRA QUADRANGLE 3 belonging to the genus Smithiphyllum, the age indicator lithologically and faunally to the upper part of the for the Frasnian age noted in Dutro and Patton (1982, Cheeneetnuk Limestone of the McGrath A-4 and A-5 p. H21), or the reported accompanying stick-like quadrangles, the Cheeneetnuk comprising the highest stromatoporoid Amphipora, were recovered. Instead, a Devonian carbonate unit recognized in Nixon Fork diverse fauna of Emsian age corals was found at the site subterrane strata in that region. where Smithiphyllum was indicated to occur. The The Devonian strata at “Reef Ridge” and the stromatoporoid Amphipora has a long stratigraphic range surrounding area have long been of interest to mineral throughout much of the Devonian, and is commonly exploration companies because of the occurrence of found in both Emsian and Eifelian age strata of the Mississippi Valley-type lead–zinc deposits. It is our aim Medfra Quadrangle. Based on field studies totaling at in this paper to provide a more detailed description of the least four weeks in duration, the youngest beds we have Lower Devonian stratigraphic succession of the Medfra recognized in the Devonian succession in the Medfra B-3 Quadrangle, in order to better constrain the age of the B-3 and B-4 quadrangles are Eifelian (early Middle intervals hosting the lead–zinc deposits in this region. Devonian) age limestones. These beds are similar both

63°24’ 154°22’

22 23 Reference section

Type section “Reef Ridge”

27 26

Figure 2. Map showing the location of the type and reference sections of the Soda Creek Limestone along “Reef Ridge” in sec. 23 and northern part of sec. 26, T. 24 S., R. 23 E., Medfra B-3 Quadrangle. Base is at the north end of both the type and reference sections. Contour interval 100 feet. Base map from U.S. Geological Survey, 1955, Medfra B-3 1:63,360-scale topographic map. 4SHORT NOTES ON ALASKA GEOLOGY 1999 BLODGETT AND OTHERS

SODA CREEK LIMESTONE (NEW NAME) northern section is situated along the same ridge in the NE¼ SW¼ sec. 23, T. 24 S., R. 23 E., Medfra B-3 Quad- The name “Soda Creek Limestone” is here assigned rangle (see fig. 2 for location), and is 157.0 m (515 ft) to a distinctive succession of richly fossiliferous, upper thick. The formation derives its name from nearby Soda Lower Devonian (Emsian) limestone and minor shale Creek, a tributary of the North Fork of the Kuskokwim exposed throughout much of the Medfra B-3 River, which is south and east of the type section. Quadrangle. The new unit is 149.3 m (490 ft) thick at its The lower contact of the Soda Creek Limestone with type section, which is situated along a north-trending the underlying unnamed dolostone-dominated unit ridge in the SE¼ SW¼ sec. 23, T. 24 S., R. 23 E., Medfra (fig. 5) is sharp and conformable and can be well B-3 Quadrangle (see fig. 2 for location). The unit is observed at the base of both sections of the unit along the repeated once along the ridge due to imbricate low-angle crest of “Reef Ridge.” The underlying dolostone unit is thrust faulting (figs. 3, 4). The southern section is composed of thin- to thick-bedded light-yellowish gray designated as the type section, whereas the northern weathering (medium gray on fresh surfaces), dolo- section is designated as the reference section. The mudstone to dolowackestone and lime mudstone. The

Figure 3. Geologic map of “Reef Ridge,” showing distribution of geologic unit in the vicinity of the type and reference sections of the Soda Creek Limestone. Map includes all or parts of secs. 22–23 and 26–27 of T. 24 S., R. 23 E., Medfra B-3 Quad- rangle. Contour interval 500 ft. Base map from U.S. Geological Survey, 1955, Medfra B-3 1:63,360-scale topographic map.

Dark to light gray weathering dolostone with lenses of silicified fossil (Amphipora, small brachiopods, and molluscs) (late Early Devonian, post-early Emsian)

SODA CREEK LIMESTONE (early Emsian, late Early Devonian)

Interbedded dolostone and limestone, weathering light to dark gray and buff in color (pre-Emsian) SODA CREEK LIMESTONE, A NEW UPPER LOWER DEVONIAN ORMATION IN THE MEDRA QUADRANGLE 5 dolostone underlying the Soda Creek Limestone is a deposition of the underlying unit and the Soda Creek bioclastic mudstone to wackestone with resistant, wavy Limestone. dolomite stringers roughly parallel with the bedding. The Soda Creek Limestone is composed predomi- Bedding is thin overall, varying from 0.1–0.3 m (0.3– nantly of dark gray limestone (ranging from packstone 1.0 ft). Large leperditiid ostracodes are locally present in to wackestone, with minor mudstone), but with the underlying unnamed unit. The uppermost bed of the prominent, thinner, recessive, weathering intervals of underlying unit in contact with the Soda Creek shale and shaly limestone. The unit at its type section is Limestone is a rudstone composed of branching, highly 149.3 m (490 ft) thick (fig. 6). Abundant free weathering digitate favositid tabulate corals. This bed varies from fossils, notably brachiopods and corals, are readily 0.3–0.9 m (1–3 ft) in thickness as a result of hardground available in the shaly, recessive intervals. The Soda formation prior to deposition of the overlying Soda Creek Limestone is primarily carbonate, but there are Creek Limestone. Truncation of bioclasts and sharp four significant occurrences of shaly interbeds which erosional relief occurs along strike. This disconformity create saddles in “Reef Ridge,” as well as in other represents a hiatus of uncertain length between the outcrops of the unit regionally. The uppermost shaly

Figure 4. Ridge with type and reference sections of the Soda Creek Limestone along “Reef Ridge.” View is from the northeast.

Figure 5. View looking south showing base of the type section of the Soda Creek Limestone. 6SHORT NOTES ON ALASKA GEOLOGY 1999 BLODGETT AND OTHERS

Unnamed dolostone interval is the most prominent and is approximately Covered 18.3 m (60 ft) thick. The limestones of Soda Creek contain an abundant and diverse fauna. In outcrop, fossiliferous mudstones are most common, but there are also wackestones, packstones, rudstones, floatstones, and Bioclastic packstone and wackestone with bafflestones. Thin sections reveal that many of the bryozoans and stromatoporoids mudstones are actually peloidal packstones and grainstones. Contacts between the limestones and shales are primarily sharp, but some contacts do show gradation. Some of the limestones are thin-bedded and less resistant and have a substantial argillaceous Fossiliferous shale component. Yellow marly intervals and individual beds also occur. What makes this formation so Fossiliferous shale diagnostic and gives name to the ridge itself is the abundance of large reef-like corals and stroma- toporoids. The tabulate corals are digitate to Fossiliferous shale and shaly limestone hemispherical, and include branches 5.0 cm (2 in.) in diameter and heads varying in diameter up to 0.3 m (1 ft), rarely 1.0 m (3 ft). The bulbous heads are commonly in place (fig. 7). The branching forms are Mudstone to wackestone rarely in place, commonly overturned and reworked. Fossilerous shale and shaly limestone The stromatoporoids are commonly bulbous and hemispherical. They are commonly 0.15–0.3 m (0.5–1.0 ft) in diameter. These coral and stroma- toporoid heads sometimes occur isolated in beds or Bioclastic wackestone and packstone form beds continuous along strike. Solitary rugose corals are also common and occur with the other “reefal” forms. The contact of the Soda Creek Limestone with the Fossiliferous shale and shaly limestone overlying unnamed dolostone unit is sharp, but conformable. The upper contact in the type section is poorly exposed in a saddle as the resistant carbonate Bioclastic wackestone and packsotne appears to grade into the overlying, non-resistant dolomitic formation (fig. 8). Dolostones of the Fossiliferous shale and shaly limestone Quartz lithic bioclastic wackestone overlying unnamed unit vary from medium to dark Fossiliferous shale and shaly limestone gray, and are locally black in color. These strata are typically well-bedded, but commonly contain solution-collapse breccia. The stick-like stroma- toporoid Amphipora is locally common in the unit. Some zones of cherty silicification occur in the lower Bioclastic wackestone and packstone part of this unit, and commonly contain silicified megafossils, consisting primarily of small-sized brachiopods and molluscs. The dolostone unit was not measured, but it appears to have a minimum thickness of 200 m. The lower part of the unnamed, Shale and shaly limestone overlying dolostone unit is of major interest to Bioclastic packstone mineral company geologists, because it is typically Shale and shaly limestone the host horizon for lead–zinc deposits in the region. Bioclastic peloidal packstone to grainstone The unnamed dolostone unit is succeeded by a succession of limestones bearing Eifelian (early

Figure 6. Stratigraphic column of the type section of Laminated to wavy mudstone the Soda Creek Limestone. SODA CREEK LIMESTONE, A NEW UPPER LOWER DEVONIAN ORMATION IN THE MEDRA QUADRANGLE 7

Figure 7. Large favositid tabulate coral head, typical of the more carbonate-rich intervals within the Soda Creek Limestone.

Figure 8. View looking north showing top of the type section of the Soda Creek Limestone.

Middle Devonian) age megafossils. This limestone unit relatively shallow-water paleoenvironments. However, forms the uppermost part of the Devonian carbonate although this formation appears to be of shallow-water succession in the Medfra B-3 and B-4 quadrangles, and origin, regionally it represents an interval of slightly is equivalent in age and lithology to the Cheeneetnuk deeper water sedimentation than that found in both the Limestone of Blodgett and Gilbert (1983) defined to the underlying and overlying units. Both of these units are south in the McGrath Quadrangle. dominated by dolostone, lack shaly interbeds and are notably lacking in well-developed open-marine faunas. ENVIRONMENT O DEPOSITION This is especially evident in the overlying dolostone unit, which commonly contains locally abundant Amphipora. The presence of an open-marine, diverse fauna This stick-like stromatoporoid genus is thought by many throughout the formation, especially such stenohaline workers to be most common in semi-restricted or forms as brachiopods and echinoderms, suggests that the restricted lagoonal environments. In summary, it appears unit was deposited under normal marine conditions. The that the Soda Creek Limestone represents a local presence of numerous, but poorly preserved gastropods, transgressive-regressive cycle within the upper part of and also locally large leperditiid ostracodes indicate the Lower Devonian stratigraphic succession. 8SHORT NOTES ON ALASKA GEOLOGY 1999 BLODGETT AND OTHERS

AGE and the Alaska Division of Geological & Geophysical Surveys (DGGS) for site visits to the type section and Faunal age control is amply provided by the surrounding area in the years of 1983–1985, and 1991. abundant megafossils and conodonts found throughout Field work in 1994 was supported by National the unit. Megafossils are particularly abundant in the Geographic Society Research Grant 5188-94. We are more shaly intervals, and include brachiopods, rugose grateful to Bill Beebe, Area Forester, Alaska Department and tabulate corals, echinoderm ossicles, leperdiitid of Natural Resources, Division of Forestry, McGrath, ostracodes, bivalves, gastropods, and trilobites. Favositid Alaska, for arranging helicopter support that permitted tabulate corals are especially notable in the more us to conduct our field study in the Reef Ridge area carbonate rich intervals (fig. 7). Their large size (some up during the summer of 1994. We also thank David L. to 1 m in diameter) and abundance, together with the LePain and Arthur J. Boucot for their reviews of this abundant weathered-free solitary rugose corals of the manuscript. shaly intervals, provide the origin of the informal term “Reef Ridge” for the ridge bearing the type and reference REERENCES sections. Brachiopods include over 30 species, of which the Blodgett, R.B., 1992, Taxonomy and paleobiogeo- most common elements are rhynchonellids (notably graphic affinities of an early Middle Devonian uncinulids). Genera present include Plicogypa, (Eifelian) gastropod faunule from the Livengood Stenorhynchia, Taimyrrhynx, “Uncinulus”, Nordo- quadrangle, east-central Alaska: Palaeontographica toechia?, Spinatrypa, Nucleospira, Protathyris, Abteilung A, v. 221, p. 125–168. Howellella, and Aldanispirifer. Diagnostic species are ———1998, Emsian (late Early Devonian) fossils Plicogypa cf. kayseri (Peetz), Taimyrrhynx taimyrica indicate a Siberian origin for the Farewell terrane, in (Nikiforova), “Uncinulus” polaris Nikiforova, and Clough, J.G., and Larson, F., eds., Short Notes on Howellella yacutica Alekseeva. The latter three species Alaska Geology 1997: Alaska Division of show their closest affinities with faunas from Arctic Geological & Geophysical Surveys Professional Russia (Novaya Zemlya, Taimyr, and Kolyma). Report 118, p. 53–61. Conodonts were recovered only from the lower half Blodgett, R.B., and Boucot, A.J., 1999, Late Early of the type section and indicate an early Emsian age Devonian (Late Emsian) eospiriferinid brachiopods (Polygnathus dehiscens Zone). These include the from Shellabarger Pass, south-central Alaska and following species: Polygnathus eberleini, Ozarkodina their biogeographic importance; further evidence for cf. remscheidensis at 21.3 m; Icriodus taimyricus, a Siberian origin of the Farewell and allied Alaskan accreted terranes: Senckenbergiana lethaea, v. 79, Pandorinellina exigua philipi, and O. cf. remscheidensis no. 1, p. 209–221. at 30.5 m; P. exigua philipi at 46.9–48.5 m; and Icriodus Blodgett, R.B., and Brease, P.F., 1997, Emsian (late Early taimyricus at 54.6 m. Devonian) brachiopods from Shellabarger Pass, Rugose corals from 21.3 to 57.3 m above the base of Talkeetna C-6 quadrangle, Denali National Park, the type section are dominated by relatively undiagnostic Alaska indicate Siberian origin for Farewell terrane: Pseudoamplexus altaicus, Lythophyllum spp., and rare Geological Society of America Abstracts with Zonophyllum sp. From 65.2 to 111.9 m corals include Programs, v. 29, no. 5, p. 5. Rhizophyllum schischkaticum, Lythophyllum spp., and Blodgett, R.B., and Gilbert, W.G., 1983, The Pseudamplexus altaicus, new species of Zelophyllia, Cheeneetnuk Limestone; A new Early(?) to Middle Acanthophyllum and a new genus. Although Devonian formation in the McGrath A-4 and A-5 undescribed, these higher faunas are related to early quadrangles, west-central Alaska: Alaska Division of Emsian coral faunas from the Kolyma region of Geological & Geophysical Surveys Professional northeastern Siberia. A weakly colonial version of the Report 85, 6 p, 1 sheet, scale 1:63,360. new Acanthophyllum species, occurring at 123.7 m, is Blodgett, R.B., and Johnson, J.G., 1992, Early Middle the only colonial rugosan from the type section. Devonian (Eifelian) gastropods of central Nevada: Palaeontographica Abteilung A, v. 222, p. 85–139. ACKNOWLEDGMENTS Blodgett, R.B., and Rohr, D.M., 1989, Two new Devonian spine-bearing pleurotomariacean D.M. Rohr thanks the National Geographic Society gastropod genera from Alaska: Journal of Committee for Research and Exploration for supporting Paleontology, v. 63, p. 47–52. the 1994 summer field work conducted in the vicinity of Blodgett, R.B., Rohr, D.M., and Boucot, A.J., 1988, Reef Ridge. R.B. Blodgett thanks Patiño, Inc. for Lower Devonian gastropod biogeography of the introducing him to the geology of this region in 1981; he Western Hemisphere, in McMillan, N.J., Embry, also thanks Sohio Oil Company, Union Oil Company, A.F., Glass, D.J., eds., Devonian of the World: SODA CREEK LIMESTONE, A NEW UPPER LOWER DEVONIAN ORMATION IN THE MEDRA QUADRANGLE 9

Canadian Society of Petroleum Geologists Memoir Geologic Studies in Alaska by the U.S. Geological 14, v. 3, p. 285–305. Survey, 1991: U.S. Geological Survey Bulletin 2041, Blodgett, R.B., Savage, N.M., Pedder, A.E.H., and Rohr, p. 186–201. D.M., 1995, Biostratigraphy of an Upper Lower Patton, W.W., Jr., 1978, Juxtaposed continental and Devonian (Emsian) limestone unit at “Reef Ridge”, oceanic-island arc terranes in the Medfra quadrangle, Medfra B-3 quadrangle, west-central Alaska: west-central Alaska, in Johnson, K.M., ed., The Geological Society of America Abstracts with United States Geological Survey in Alaska: Programs, v. 27, no. 5, p. 6. Accomplishments during 1977: U.S. Geological Blodgett, R.B., Sullivan, R., Clough, J.G., and LePain, Survey Circular 772-B, p. B38–B39. D.L., 1999, Paleozoic paleontology of the Holitna Patton, W.W., Jr., Moll, E.J., Dutro, J.T., Jr., Silberman, Lowland, southwest Alaska: Geological Society of M.L., and Chapman, R.M., 1980. Preliminary America Abstracts with Programs, v. 31, no. 6, geologic map of the Medfra Quadrangle, Alaska: p. A39. U.S. Geological Survey Open-File Report 80-811A, Chalmers, R.W., Measures, E.A., Rohr, D.M., and 1 sheet, scale 1:250,000. Blodgett, R.B., 1995, Depositional environments of Rohr, D.M., 1993, Middle Ordovician carrier shell an upper Lower Devonian (Emsian) limestone unit at Lytospira (Mollusca, Gastropoda) from Alaska: “Reef Ridge,” Medfra B-3 quadrangle, west-central Journal of Paleontology, v. 67, p. 959–962. Alaska: Geological Society of America Abstracts Rohr, D.M., and Blodgett, R.B., 1988, First occurrence with Programs, v. 27, no. 5, p. 9. of Helicotoma Salter (Gastropoda) from the Decker, John, Bergman, S.C., Blodgett, R.B., Box, S.E., Ordovician of Alaska: Journal of Paleontology, v. 62, Bundtzen, T.K., Clough, J.G., Coonrad, W.L., p. 304–306. Gilbert, W.G., Miller, M.L., Murphy, J.M., Robinson, Rohr, D.M., Dutro, J.T., Jr., and R.B. Blodgett, 1991, M.S., and Wallace, W.K., 1994, Geology of Gastropods and brachiopods from the Ordovician southwestern Alaska, in Plafker, George, and Berg, Telsitna Formation, northern Kuskokwim H.C., eds., The Geology of Alaska: Boulder, Mountains, west-central Alaska: Sixth International Colorado, Geological Society of America, The Symposium on the Ordovician System - Abstracts - Geology of North America, v. G-1, p. 285–310. University of Sydney, Sydney, Australia, July 15–19, Dutro, J.T., Jr., and Patton, W.W., Jr., 1982, New 1991. Australia Bureau of Mineral Resources, Paleozoic platform formations in the northern Geology and Geophysical Record 1991/47, p. 29. Kuskokwim Mountains, west-central Alaska: U.S. ———1992, Gastropods and brachiopods from the Geological Survey Bulletin 1529-H, p. H13–H22. Ordovician Telsitna Formation, northern Frýda, J., and Blodgett, R.B., 1998, Two new cirroidean Kuskokwim Mountains, west-central Alaska, in genera (Vetigastropoda, Archaeogastropoda) from Webby, B.D., and Laurie, J.R., eds., Global the Emsian (late Early Devonian) of Alaska with perspectives on Ordovician Geology: Proceedings of notes on the early phylogeny of Cirroidea: Journal of the Sixth International Symposium on the Paleontology, v. 72, p. 265–273. Ordovician System, Sydney, Australia. Balkema Hahn, G., and Hahn, R., 1993. Neue Trilobiten-Funde Press, p. 499–512. aus dem Karbon und Perm Alaskas: Geologica et Rohr, D.M., and Gubanov, A.P., 1997, Macluritid Palaeontologica, v. 27, p.141–163. opercula (Gastropoda) from the Middle Ordovician Measures, E.A., Blodgett, R.B., and Rohr, D.M., 1992a, of Siberia and Alaska: Journal of Paleontology, v. 71, Depositional setting and fauna of Middle Ordovician p. 394–400. rocks of the Telsitna Formation, northern Savage, N.M., Rohr, D.M., and Blodgett, R.B., 1995, Kuskokwim Mountains, Alaska: Geological Society Late Silurian condonts from the Medfra B-4 of America Abstracts with Programs, v. 24, no. 5, quadrangle, west-central Alaska: Geological Society p. 70. of America Abstracts with Programs, v. 27, no. 5, Measures, E.A., Rohr, D.M., and Blodgett, R.B., 1992b, p. 76. Depositional environments and some aspects of the Stock, C.W., 1981, Cliefdenella alaskanensis n. sp. fauna of Middle Ordovician rocks of the Telsitna (Stromatoporoidea) from the Middle/Upper Formation, northern Kuskokwim Mountains, Alaska, Ordovician of central Alaska: Journal of in Bradley, D.C., and Dusel-Bacon, C., eds., Paleontology, v. 55, p. 998–1005.

DUPLEX STRUCTURE AND PALEOCENE DISPLACEMENT O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY, NORTH-CENTRAL BROOKS RANGE Reia M. Chmielowski,1 Wesley K. Wallace,2 and Paul B. O’Sullivan3

ABSTRACT

The Toyuk thrust zone is a regional tectonic boundary in the central Brooks Range of northern Alaska that has been interpreted previously as a single large-displacement fault. Just west of the Dalton Highway, the thrust zone is defined by several north-vergent imbricate thrust faults. These faults have cut detachment folds formed in the competent Kanayut Conglomerate between detachments in the underlying Hunt Fork Shale and overlying Kayak Shale, thereby forming a duplex. The exposed structural geometry suggests two end- member models of duplex geometry. In model one, each linking thrust has roughly the same small displacement and the roof thrust is overlain by a normal stratigraphic succession. In model two, the uppermost and hindmost fault has greater displacement than the other faults and so forms a duplex roof above which the stratigraphic section is duplicated. Discrimination between these models is not possible because the duplex roof has been eroded in the field area.

Apatite fission-track analyses of samples collected within the Toyuk thrust zone record two distinct episodes of cooling interpreted to represent unroofing in response to tectonic deformation. The initial episode occurred at ~100 Ma, while the later episode occurred at ~60 Ma. These two cooling/denudation events have been documented previously in the region, but the samples from the thrust zone are the first to document displacement on a specific fault during the ~60 Ma event so far south of the front of the central Brooks Range. The results suggest that at least part of the growth of the Toyuk thrust zone accommodated structural thickening within the orogenic wedge coeval with deformation at the range front.

INTRODUCTION

The folds in many fold-and-thrust belts have been northern Brooks Range (Wallace and others, 1997). These interpreted to be fault-bend or fault-propagation folds, in structures form duplexes in the Kanayut Conglomerate, which faulting predates or is synchronous with folding with a floor thrust in the Hunt Fork Shale and a roof thrust (Suppe, 1983; Jamison, 1987; Suppe and Medwedeff, in the Kayak Shale, both of which are relatively 1990). However, in many areas those models may not incompetent units. apply. Recent studies in the Brooks Range of northern Previous workers have generally assumed that the Alaska have documented the formation of duplexes in regional boundary between exposures dominated by the which folds formed before being cut by thrust faults Hunt Fork Shale to the south and the Kanayut (Wallace and others, 1997; Homza and Wallace, 1991; Conglomerate to the north (fig. 2), is a single thrust fault, Homza, 1992). This study examines the structural the Toyuk thrust (Moore and others 1989, 1997; Kelley geometry and evolution of the Toyuk thrust zone, an and Brosgé, 1995). This study explores the alternative example of just such a duplex within the Endicott hypothesis that the regional boundary between the Hunt Mountains allochthon in the Brooks Range (fig. 1). Fork and the Kanayut is not simply a single thrust fault The lower part of the Endicott Mountains allochthon that is continuous along strike, but instead reflects the consists dominantly of the Upper Devonian Hunt Fork local intersection of the erosion surface with a duplex in Shale, the Upper Devonian to Lower Mississippian (?) the Kanayut, referred to here as the Toyuk thrust zone Kanayut Conglomerate, and the Mississippian Kayak (Wallace and others, 1997). This hypothesis allows an Shale (Brosgé and others, 1979a, 1979b; Mull and others, estimate to be made of the minimum possible displace- 1989; Moore and others, 1989, 1994a, 1997). The contrast ment across the zone. in competency between the Kanayut and its overlying Detailed geologic mapping (1:25,000 scale) of an and underlying units has resulted in formation of a approximately 23-square-km portion of the Toyuk thrust combination of map-scale detachment folds and thrust zone was completed during an 8-week field season during faults within the Kanayut during the evolution of the the summer of 1995. The field area is north of Atigun Pass

1Geology Department, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, Alaska 99508-8338. Now at: 1910 Mt. Vernon Ct., Mountain View, California 94040. Email for Reia M. Chmielowski: [email protected] 2Department of Geology & Geophysics, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, Alaska 99775-5780. 3LaTrobe University, Department of Geology, Bundoora, VIC 3083 Australia.

11 12 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS between the east and west forks of the Atigun River along REGIONAL GEOLOGY the northern flanks of James Dalton Mountain just south of the west fork of the Atigun River (fig. 2). The Dalton Most of the Brooks Range is part of the Arctic Alaska Highway is about 9.6 km due east of the area. The map- terrane (Jones and others, 1987; Silberling and others, ping concentrated on precisely delineating folds and 1994). The area of this study is in the northern part of the faults in the area and determining their relationship to the central Brooks Range, and is wholly within the Endicott stratigraphy. Several parallel steep gullies on the north Mountains allochthon, one of the subterranes of the flank of James Dalton Mountain are perpendicular to the Arctic Alaska terrane (Moore and others, 1994a) (fig. 1). strike of the faults. These gullies provide a three- A number of regional cross sections illustrate the dimensional picture of the structure, which facilitated the structural style and stratigraphy of the Endicott construction of cross sections. Mountains allochthon and the northern Brooks Range To constrain the timing of activity on the faults, a (Mull and others, 1987; Oldow and others, 1987; Grantz series of samples for apatite fission-track analysis were and others, 1991; Moore and others, 1994b). collected from outcrop localities spanning the thrust zone DEORMATION HISTORY (fig. 3). This approach potentially provides a powerful tool for elucidating the history of movement of major The initial phase of the Brookian orogeny began with faults, especially in areas of reactivation where cross- the collision of an intraoceanic arc represented by the cutting relations are lacking (O’Sullivan and others, Angayucham and Koyukuk terranes with the south- 1998a). facing passive continental margin of the Arctic Alaska

Figure 1. Location map, showing the terranes and major tectonic elements of northern Alaska as well as the Dalton Highway. Location of geologic map (fig. 2) is outlined. Modified from figure 2 of Wallace and others (1997). DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 13 terrane in Middle Jurassic to Early Cretaceous time allochthon, the units within the allochthon have (Moore and others, 1994a). The partial subduction of the undergone significant internal shortening via map-scale Arctic Alaska terrane involved hundreds of kilometers of thrust faults and folds. shortening that resulted in northward delamination and The second phase of the Brookian orogeny, from mid- imbrication of the continental superstructure to form a Cretaceous and into the Neogene, involved tens of stacked series of allochthons (Moore and others, 1994a). kilometers of post-collisional shortening that resulted in The Endicott Mountains allochthon is the structurally uplift and unroofing along the main axis of the Brooks lowest of these allochthons (fig. 1) (Mull, 1982; Mayfield Range and its northern foothills (Moore and others, and others, 1988). 1994a). Apatite fission-track cooling ages of ~100 Ma, ~60 The Endicott Mountains allochthon was displaced at Ma, and ~24 Ma from the Brooks Range and its foothills least 88 km northward over less deformed rocks that suggest that discrete deformational events contributed compose the North Slope parautochthon (Mull and to that unroofing (O’Sullivan, 1996; O’Sullivan and others, 1989). In addition to the displacement of the entire others, 1997, 1998b).

Figure 2. Generalized geologic map of the north-central Brooks Range, showing distribution of stratigraphic units and structures for the portion of the Endicott Mountains allochthon along the Dalton Highway. The study area is outlined. Modified from figure 3 of Wallace and others (1997). 14 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS

Figure 3. Simplified geologic map of the field area. Shows the location of cross-section lines and fission-track samples, and identifies the faults and thrust-bound panels. DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 15

STRUCTURAL STYLE THE TOYUK THRUST ZONE

The structural style of the Endicott Mountains Although it is mentioned extensively in the literature, allochthon is fundamentally influenced by its mechanical the Toyuk thrust has only been loosely defined and stratigraphy. Relatively incompetent shale units serve as identified in previous works. The name originally applied detachment intervals and alternate with competent units to a single thrust fault which is exposed on the southern of conglomerate and sandstone or of limestone (Brosgé flanks of Toyuk Mountain, near Anaktuvuk Pass (Porter, and others, 1979a; Mull and Adams, 1989; Wallace and 1966). However, the name now applies to a regional others, 1997). The relative thickness and competency of boundary within the Endicott Mountains allochthon these units control the size and location of folds and that separates exposures that consist predominantly of thrusts, as well as the spacing and orientation of the Hunt Fork Shale to the south and Kanayut Conglomerate ramps and flats within the faults. The rocks of the to the north (Brosgé and others, 1979b; Mull and Adams, allochthon are stacked in south-dipping thrust fault- 1989). The Toyuk thrust zone has been mapped ~300 km bounded sheets, with displacements ranging from tens of to the west and ~140 km to the east of the field area. The meters to a few kilometers, and map distances between Toyuk thrust zone tends to be thought of as a single faults of about 0.5 to 5 km (fig. 2). Map-scale north- traceable fault along strike, but existing mapping shows vergent folds and smaller parasitic folds reflect structural that it actually is a zone that consists of multiple faults thickening within the sheets. Leading hanging-wall along its length (Brosgé and others, 1979a, b; Mull, 1989; anticlines truncated by underlying thrust faults are Kelley, 1990; Mull and others, 1994; Wallace and others, common and trailing footwall synclines are exposed less 1997). This zone appears to mark a regional change in commonly. However, large unbroken fold pairs are seen depositional facies within the Endicott Mountains only locally within the sheets. The folds in the Kanayut allochthon, most notably the change between the Conglomerate, a thick, competent unit, are larger in southern facies of the Kanayut Conglomerate and the amplitude and wavelength than those in the Hunt Fork thicker, more proximal northern facies (Kelley and Brosgé, Shale, an incompetent unit. Within the southern part of 1995; Moore and others, 1997). However, the exact the allochthon, including the study area, folds and thrust location and character of the facies change with respect faults within the competent Kanayut Conglomerate are to the Toyuk thrust zone has not been determined in detail bounded below by a detachment in the Hunt Fork Shale along the length of the zone. At different places along and above by a detachment in the Kayak Shale (Wallace strike, it could lie within the fault zone itself or nearby to and others, 1997). the north or south of the zone. The map-scale folds in the Kanayut Conglomerate The Toyuk thrust zone has been suggested to be a have been interpreted to have originated as detachment major large-displacement thrust within the Endicott folds (Wallace and others, 1997). The contrast in com- Mountains allochthon on the basis of its length and petency between the Kanayut Conglomerate and the because it separates different facies in the Kanayut underlying Hunt Fork facilitated the formation of these (Moore and others, 1997). Minimum displacements of 18 folds. Although folds that have not been cut by thrusts km (Moore and others, 1997), 8 km (Kelley and Brosgé, are rare, the abundant folds cut by thrusts are similar in 1995), and 5 km (Wallace and others, 1997) have been character to thrust-truncated detachment folds that reported for individual faults in the zone, from west to display a transition into unfaulted detachment folds in the east. However, near the Dalton Highway, Wallace and northeastern Brooks Range (Homza, 1992; Wallace, 1993). others (1997) described the zone to consist of a series of The thrust faults cut the steep limbs of hanging-wall different thrust faults, none of which continues along anticlines and footwall synclines at a high angle to strike for the entire length of the fault zone and that bedding. These thrust faults form a duplex (Wallace and juxtapose Hunt Fork Shale to the south against Kanayut others, 1997). The thrust flats, where exposed, are in the Conglomerate to the north (fig. 2). They interpret this to Hunt Fork Shale and Kayak Shale so that the reflect different horses in a duplex of thrust-truncated stratigraphic section included within the thrust sheets detachment folds being revealed by changes in the level is generally limited to Hunt Fork Shale through Kayak of erosion along strike. This interpretation requires only Shale. Where the overlying Lisburne Group is not eroded, relatively small displacement on most of the individual disharmony exists in structures across the Kayak. These thrust faults in the duplex, although it does not preclude characteristics suggest that the thrust faults broke a larger displacement on the southern, structurally through (truncated) the steep to overturned limbs of highest fault. asymmetrical folds in the Kanayut Conglomerate to form a duplex of thrust-truncated folds between the Hunt Fork Shale and the Kayak Shale (Wallace and others, 1997). 16 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS

LOCAL GEOLOGY synclines are exposed in the field area, although they have been observed elsewhere in the region (Wallace and MECHANICAL STRATIGRAPHY others, 1997). The stratigraphic units in the study include, from No direct evidence indicates the type of folds present bottom to top, the Upper Devonian Hunt Fork Shale, in the field area. However, the contrast in competency Upper Devonian Noatak Sandstone, Upper Devonian and between the units, the thickening of the incompetent Lower Mississippian (?) Kanayut Conglomerate, and units, the trains of multiple folds in several panels, and the Lower Mississippian Kayak Shale. The relative compe- indication that at least some of the folds pre-date the tency of the units strongly influences the structure of the faults all favor the interpretation that these folds formed area. The regional and local characteristics of these units as detachment folds. The regional presence of detach- are summarized in table 1. ment folds (Wallace, 1993; Wallace and others, 1997) further supports this interpretation. Fault-bend folding STRUCTURAL ORGANIZATION O and fault-propagation folding are less likely alternatives THE IELD AREA because of the presence of an underlying incompetent unit, the presence of fold trains, and the local decapitation The field area is divided into panels bounded by of folds, which indicates that the folds pre-date the faults. thrust faults that strike between N 65° E and N 77° E and In the study area, the folds are truncated by faults in dip gently (15° to 30°) to the south (figs. 3, 4; table 2). The two distinct manners. In panels B and Z (figs. 3, 4) the faults are designated from bottom to top by numbers 1 to steep, short limbs of anticlines are clearly truncated by the 3, except for the southernmost thrust to the west, which underlying fault. The other manner of fold truncation is is labeled “X” because its relationship to fault 3 is unclear present only in panel B, where the tops of the folds have in the field area. However, fault X is shown by Wallace and been cut off by the overlying fault (fig. 4). In this case, others (1997) to cut across fault 3. Faults 2A and 2B are both the back limbs and forelimbs of several folds have here interpreted to be splays of the same fault. The panels been cut. While it could be argued that folding was con- separated by these faults are designated A, B, C1, C2, D, temporaneous with faulting where only the forelimb is and Z, from lowest to highest (north to south). truncated, as in fault-propagation folds (Suppe and Medwedeff, 1990; Mitra, 1990), the decapitation of the GEOMETRY AND SEQUENCE O STRUCTURES fold triplet in panel B (fig. 3; fig. 4 sec. C–C’) clearly Thrust Truncation of Detachment olds shows that those folds pre-date the fault that cut them. Detachment folds may develop in deformed areas The truncation of bedding at high angles and the where incompetent units underlie competent units decapitation of some folds by thrust faults both suggest (Poblet and McClay, 1996; Homza and Wallace, 1997). that pre-existing folds were cut by thrust faults. These Contraction causes the competent layers to fold and the observations are consistent with the interpretation that incompetent layers to flow into the core of the fold above the folds in the area are thrust-truncated detachment folds, a detachment that bounds the fold below. This is in as has also been suggested by Wallace and others (1997). contrast to fault-bend folds, in which folding is in response to bends in the underlying fault (Suppe, 1983), Duplex Thrust System or fault-propagation folds, in which folding is in response The thrust faults cut and repeat a limited stratigraphic to deformation preceding a propagating ramp tip (Suppe interval (Dhf to Mky). Each panel consists of folded and Medwedeff, 1990; Mitra, 1990). As detachment folds strata. Fault 3 parallels bedding in Hunt Fork at the base tighten, they may be truncated by thrust faults, which of panel D, and fault 1 parallels the Kanayut–Kayak leave anticlines in the hanging walls and synclines in the contact at the top of panel A (fig. 4). These relations footwalls (Morley, 1994; Liu and Dixon, 1995; Wallace and suggest that the thrusts of the area are bounded below Homza, 1996, 1997). Further shortening may cause the by a floor thrust in Hunt Fork and above by a roof thrust process to be repeated, which can result in a structural in Kayak, although neither are exposed in the study area. stack of fault-bounded panels of folded rock. In this interpretation, the structure of the area constitutes Figure 5 illustrates a schematic example of detach- a duplex of thrust-truncated detachment folds (fig. 6B), ment folding and the truncation of a fold by a thrust fault. which is consistent with a similar interpretation for the An anticline in the hanging wall and a syncline in the structure of the region proposed by Wallace and others footwall result from displacement on a thrust fault that has (1997). The interpretation that the horses in this duplex cut the steep limb of an asymmetrical fold pair. Most of the contain pre-existing folds that were cut by the linking panels in the field area contain a series of folds. The thrusts of the duplex differs from more familiar duplex foremost anticline of each series of folds has been interpretations, in which folds within the duplex are truncated in this manner (fig. 4). No truncated footwall limited to fault-bend folds or, rarely, fault-propagation DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 17 Comments include either or both facies. Forms flats of major thrust faults. In fault-bounded slivers in panel C1. thrust faults and cores folds in Kanayut. Rock types Structure estone marker beds in its upper part by small folds and thrust faults. sandstone. structural thickening. Forms flats of rusty orange nodules.a single shale exposure as the underlying Dhf unit (panel C2). Noatak is treated as However, part of the overlying Kanayut for development of models. and ramps of thrust faults.of identification on based area Black fissile clay shale with rare Incompetent. Internally thickenedConglomerate, sandstone, siltstone,and shale in Ear Peak Stuver. Upper detachment unit. (Shainin Lake, which is not identified in Competent. Forms large folds and ramps of thrust faults. Competent unit. The northern facies is thicker than the Calcareous sandstone that shows southern facies. The field area foldslarge Forms Competent. field the in present Possibly Shale and subordinate thin interbedsinternalConsiderable Incompetent. unit. detachment Lower (Mky) lim (Dhf) of (Dns) Unit Summary of mechanical stratigraphy North facies divided into members: Ear Peak (MDke) (lower), Shainin Lake (middle, not identified in field area), and Stuver (MDks) (upper). Southern facies not divided into members Lower Mississippian Kayak Shale Upper Devonian to Lower Mississippian (?) Kanayut Conglomerate =undifferentiated(MDku Kanayut unless specificmember [below] is named). the field area, consists only of conglomerate and sandstone.)Upper Devonian Noatak Sandstone Upper Devonian Hunt Fork Shale is in the boundary zone between the two, and could Table 1. Table • • • • 18 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS

Figure 4. Cross sections across the Toyuk thrust zone, including locations and ages of cooling apatite fission-track samples. Sections have been lined up based on the intersection of fault 1 and the 4,500 ft elevation mark. DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 19 folds (Boyer and Elliott, 1982; Mitra, 1986; Mitra and both locally and regionally support the duplex Boyer, 1986). interpretation. Furthermore, carbonate rocks of the An alternative interpretation of the same structural Lisburne Group, exposed in an erosional remnant to the geometry would be an imbricate fan (fig. 6A). The primary northwest of the field area, are folded disharmonically difference between a duplex and an imbricate fan is the with respect to the underlying Kanayut. This relationship presence or absence of a roof thrust. An imbricate fan supports the interpretation of a roof thrust in the consists of thrusts that branch upward from a common intervening Kayak Shale (fig. 2). floor, while a duplex consists of thrusts that link a floor thrust with a roof thrust (Boyer and Elliott, 1982; McClay, MODELS OR THE DISPLACEMENT O THE TOYUK 1992). It is difficult to distinguish between the two in THRUST ZONE areas where the roof thrust has been eroded, but the Each of the panels within the study area exposes restricted range of stratigraphic section contained in each Kanayut Conglomerate and/or Kayak Shale, with the panel and the evidence for flats in Hunt Fork and Kayak exception of panel D (figs. 3, 4). This topographically and

Table 2. Summary of fault-bounded panels within the field area

Panel Thickness Upper Lower Stratigraphic units contact contact (see table 1)

A >200 m Fault 1 Not exposed MDks and Mky B 300–400 m in the eastern Fault 2A or 2B Fault 1 MDku half of field area; 600–800 m in the western half. C1 100–150 m Fault 2B Fault 2A Mky C2 500 m Fault 3 and Fault X Fault 2B MDke(?), Dns(?), and Dhf(?) D >400 m Not exposed Fault 3 Dhf Z >200 m Not exposed Fault X MDku

Figure 5 (left). Thrust truncation of detachment folds. Folds in competent conglomerate are cored by incompetent shale. Further shortening allows thrust faults to breach the forelimbs of the anticlines and to displace the folded package over the synclines. Figure 6 (below). Imbricate fan versus duplex. (A) An imbricate fan consists of thrust faults which branch upward from a single basal thrust. (B) A duplex consists of stacked panels of fault-bounded rocks. A duplex must have both a floor and a roof thrust, as well as thrusts linking the two. Pattern fills do not designate bedding. 20 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS structurally highest panel exposes only Hunt Fork displacement of this panel on fault 3. Model one shows Shale, the oldest stratigraphic unit of the area. However, similar displacements on each of the thrusts so that fault Hunt Fork is present in the hanging walls of the other 3 simply serves as another linking thrust in the duplex faults up-plunge to the east of the area (Brosgé and (fig. 7, top). Model two shows a much larger displacement others, 1979a; Wallace and others, 1997). The Toyuk on fault 3 so that panel D forms the roof of the duplex thrust zone has previously been interpreted as a major (fig. 7, bottom). large-displacement fault (Moore and others, 1997). The Since no truncated footwall synclines are exposed in duplex interpretation proposed above does not require the field area, their locations were approximated on the a large displacement, although it still allows for large models by assuming that only the folds exposed at the displacement. Multiple faults within a duplex may bring surface plus a truncated trailing syncline were present (as underlying units such as Hunt Fork into contact with in fig. 5). Similarly, a leading truncated anticline was overlying units such as Kanayut with fairly little inferred if one wasn’t exposed in the panel. The inferred displacement on any one fault, although the cumulative folds were placed such that their geometry matched other structural relief across the duplex may be large. folds in the area, and were located to give the minimum Two end-member models with different implications possible displacement on each fault. for displacement are consistent with the observations of Both models are partially restored in figure 8. Move- this study and the duplex interpretation (fig. 7). In both ment along the faults was removed by restoring each models, a duplex in the Kanayut Conglomerate is panel to the point where the bedding contacts matched bounded below by a floor thrust in Hunt Fork. The across the faults. This restoration attempts to restore each difference between the models is in the amount of fold to its pre-faulted configuration. The restorations are displacement on the highest and hindmost fault in the only approximate because some faulting likely was duplex. The exact position and character of the change contemporaneous with the folding in both cut and uncut between the northern and southern facies of the Kanayut folds. However, this restoration is still useful to determine are unknown, so two versions of each model were the minimum displacements necessary along each fault. constructed to explore the consequences for displacement The minimum displacement on each fault is given in of the different thicknesses of each facies. One version table 3. These estimates are based on the amount of fault uses a thickness measured in the northern facies (~2,600 movement necessary to restore the bedding contacts m), and the other uses a thickness determined for the across the faults. These numbers are approximate southern facies (~1,000 m) (Moore and others, 1989). because the truncated footwall synclines in each panel These models were based on cross section C–C’ are inferred and the truncated hanging-wall anticlines do (fig. 4) for several reasons. It is the section where panel not expose their upper contacts. In model two there are B is thickest and has the best constrained boundaries. It no constraints upon which to base the estimates for is relatively near a small exposure of probable Hunt Fork displacement for fault 3. Therefore I have used a “greater Shale in panel C2 (fig. 3 or see plate 1, Chmielowski, 1998) than” symbol to indicate that the value is not calculable that provides a constraint on the location of the top of at this time. that unit within the panel. Panel C1 is absent in that area, which simplifies the model by requiring only a single fault Table 3. Estimates of minimum displacement on the faults to be considered between panels B and C2. of the Toyuk thrust zonea Panels A and C2 are the best constrained. Panel A has a well defined contact between the Kanayut AB Conglomerate and the overlying Kayak Shale, and so Northern facies Southern facies approximate thicknesses for each facies of the Kanayut Fault 1 500 m 625 m were projected downward from that contact. Folds were Fault 2 800 m 750 m projected upward and downward to match the axial Fault 3 surfaces and bedding attitudes observed in the cross Model 1 450 m 1,000 m section. The elevation of the probable Hunt Fork/ Fault 3 Kanayut contact in panel C2 was used as the baseline Model 2 ≥6,300 mb ≥4,000 mb from which to project the thicknesses of overlying Kanayut. For the sake of simplicity, the Noatak Sandstone aThese values were obtained by comparison of the partially was combined with the Kanayut in these models. Since restored versions (fig. 8) with the unrestored models (fig. 7). the location of contacts in panel B is less constrained, it bNo constraints are known upon which to base estimates for displacement on fault 3 in model 2, except that they must exceed is taken to have a displacement intermediate between that the minimum values in model 1. The values given here are based of panels A and C2. Panel D is the least constrained of the on figure 7, which shows displacement if panel D overlaps faults panels; the main difference in the two models is in the 1 and 2. DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 21

MODEL ONE and 8 in which an abrupt thickness change is assumed just All of the faults in this model show minimum below fault 3 so that the northern facies is shown in the displacements (table 3). The southward increase in horses below and forward of fault 3, and the southern structural relief within the duplex is greater than the facies is shown above fault 3. Thus, the estimated increase in elevation of the erosion surface. displacements reported in table 3 that would apply to this Consequently, the erosion surface cuts down-section to version are for the southern facies for fault 3 and for the the south, and panel D is the only one that exposes a northern facies for faults 1 and 2. In this interpretation, the significant amount of Hunt Fork Shale at the surface hindmost fault that is localized at the facies change might (fig. 7, top). The minimum displacements for this model are also be expected to be the fault with the greatest much less than previously published estimates for the displacement. Although this cannot be determined within Toyuk thrust zone (Moore and others, 1997) because the the study area, it is consistent with an estimated minimum model considers only the displacement needed to account displacement of 5 km on the probable continuation of for the current configuration via a duplex of thrust- fault 3 south of the study area (Wallace and others, 1997). truncated detachment folds. APATITE ISSION-TRACK ANALYSIS MODEL TWO METHODOLOGY AND Model two differs from model one only in the amount THERMOCHRONOLOGY of displacement assumed on the southernmost and structurally highest fault, fault 3 (fig. 7, table 3). Displace- To constrain the unroofing history of the Toyuk ment on this fault must be greater than the minimum thrust zone and to relate its activity to regional Brooks assumed in model one but otherwise is unconstrained Range tectonics, apatite fission-track (AFT) data were since the hanging-wall cutoff of the Hunt Fork/Kanayut gathered from seven outcrop samples collected across a contact has been eroded. For this model, fault 3 is broad range in elevation from four different fault panels assumed to have sufficient displacement to place panel (figs. 3, 4). D over the exposed positions of faults 1 and 2 (fig. 7, Fission tracks in apatite grains preserve a record of the bottom). Thus, panel D forms the roof of the duplex, with thermal history of the host rock below temperatures of the roof thrust located between Hunt Fork in the hanging ~110°C. Thus, they are ideal for determining the timing of wall and Kayak in the footwall. This assumption yields a denudation in response to uplift of upper crustal rocks much greater displacement for fault 3 than for the other (Naeser, 1979). The length reduction of confined tracks (linking) thrusts in model two or any of the thrusts in leads to the reduction of fission track age, and the length model one (table 3). distribution of the confined tracks in apatite directly reflects its thermal history subsequent to the last time it STRATIGRAPHIC BEST-GUESS VERSION O THE cooled below ~110°C (Gleadow and others, 1986; Green MODELS and others, 1989). In this study, the AFT data have been Many more versions of the models are possible to interpreted using the system response (Green and others, represent different positions and widths of facies change 1989) based on an empirical kinetic description of across the Toyuk thrust zone in the study area, but the laboratory annealing data in Durango apatite (Laslett and available data aren’t sufficient to determine which of others, 1987). Thermal history interpretations are based these models is correct. However, the fact that the three on a quantitative treatment of annealing achieved by members of the Kanayut can be identified and mapped as forward modeling (Green and others, 1989) of track far south as fault 3 just south of the field area (Wallace, shortening and age evolution through likely thermal unpublished mapping) suggests that these rocks belong histories for an apatite composition equal to that of to the northern facies and that the facies change lies Durango apatite (0.4 weight percent chlorine [Cl]). somewhere at or to the south of fault 3, but north of The annealing behavior of fission tracks in apatite is exposures of the southern facies at Atigun Pass. This sensitive to apatite chemistry, particularly to the ratio of suggests the interpretation that the Toyuk thrust zone chlorine to fluorine (Green and others, 1986; O’Sullivan formed at the facies change (Wallace and others, 1997; and Parrish, 1995), so the Cl content of individual grains Chmielowski, 1998), with the southernmost fault in the dated in three representative samples from different fault duplex being localized at the facies change and additional panels (RMC7-6, RMC7-1, RMC8-6) was determined by horses forming within the northern facies in the footwall microprobe analysis. The results indicate that all 75 grains of this fault. Another version of the models is presented were F-apatite with Cl concentrations ranging between to illustrate this best guess as to the relation between the ~0.0 and 0.4 weight percent Cl, thus there is no evidence facies change and the thrust zone (fig. 9). This version is of major compositional variation among the different simply a composite of the versions presented in figures 7 apatite grains. These Cl concentrations are less than or 22 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 23 ly never existed, as s. Models for thrust displacement. Model one has a similar displacement on all faults. two much greater on fault 3. Partially restored sections: Fault displacement is restored to show fold shortening only. In actuality this configuration like to show fold shortening only. sections: Fault displacement is restored restored Partially some folding probably occurred after the faulting began. Each model has two versions, one with thicknesses appropriate to the northern facies of the Kanayut, some folding probably occurred after the faulting began. Each model has two versions, one with thicknesses appropriate to northern Dhf is the Hunt Fork Shale, one for the southern facies. All three members of Kanayut Conglomerate as well Noatak Sandstone are included in MDku. and Mky is the Kayak Shale. Pattern fills do not designate bedding. Each model has two versions, one with thicknesses appropriate to the northern facies of Kanayut, for southern facie Figure 8 (above). Figure 7 (page 22). 24 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS Stratigraphic best-guess version of the models. Comparison of model one (A) and model two (B) and the partially restored section (C) for each if it is assumed Stratigraphic best-guess version of the models. Comparison model one (A) and two (B) partially restored section that fault 3 is the border between thicker northern facies of Kanayut and thinner southern facies. This version intermediate between the two end- member versions of the models presented in figures 7 and 8. Figure 9. DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 25 equal to that for Durango apatite (0.4 weight percent Cl), particularly true for sample RMC8-7 with a mean length so paleotemperatures estimated by applying the Laslett of ~14.4 µm and a standard deviation of ~1.1 µm. and others (1987) model for track annealing are interpreted Therefore, these results record a major episode of rapid to be maximum values. Gallagher (1995) automated the cooling during the Paleocene at ~60 Ma. This age of modeling procedure to give a forward modeling approach cooling is consistent with the 60 ± 4 Ma age for a series that combines a Monte-Carlo simulation of numerous of samples collected east of the Dalton Highway near the possible thermal histories with statistical testing of the range front (O’Sullivan and others, 1997). outcome against the observed fission-track measure- The results from two of the samples (RMC8-3, RMC7- ments. A genetic algorithm is also used to provide rapid 6), however, show slightly shorter mean track lengths and convergence to an acceptable fit. The absolute paleo- broader track-length distributions (table 4). Modeling of temperature estimates quoted in this paper have an these results using the forward modeling approach uncertainty of ±10°C (Green and others, 1989). described by Gallagher (1995) indicates an earlier episode of cooling from temperatures >~110°C to temperatures in RESULTS AND INTERPRETATIONS the range of ~80–90°C prior to the later episode of cooling to temperatures <~50–60°C at ~60 Ma. The best-fit model Sample information and results are presented in table for timing of this earlier event suggests cooling at some 4, while the complete results are presented in Appendix time between ~90–110 Ma. The timing of this event also B of Chmielowski (1998). In figure 4, the ages are shown fits well with the previous results by O’Sullivan and on the cross sections to put them into geological context, others (1997), who suggested that a major episode of and in table 4 the samples are grouped according to the cooling/denudation occurred at ~100 ± 5 Ma in the region nearest cross-section line to illustrate the changes in age around Atigun Pass. with elevation across different parts of the area. The AFT Therefore, we believe that the AFT data indicate that ages of these samples range between 60 ± 3 and 79 ± 10 the rocks within the Toyuk thrust zone have undergone Ma and the mean track lengths range between 13.4 ± 0.1 two major episodes of relatively rapid cooling from µm and 14.4 ± 0.1 µm with standard deviations between elevated paleotemperatures: an initial period of cooling in 1.1 and 1.6 µm (all uncertainties presented below are ±1s the mid-Cretaceous at ~100 Ma, after which some samples unless otherwise stated). remained at elevated paleotemperatures between ~80 and All AFT ages were much younger than the Devonian 90°C, followed by a later Paleocene cooling at ~60 Ma to Mississippian depositional ages of the rocks, which (fig. 10). Of the two episodes of cooling, the Paleocene indicates a reduction in grain ages in response to thermal event is better constrained as the samples with the best annealing since the rocks were deposited. Furthermore, data yield the youngest ages. Evidence for the earlier the long mean track lengths (most >13.8 µm) and narrow cooling event is reflected in a few of the samples (RMC8- track length distributions (most standard deviations <1.3 3, RMC7-6) by the range of single-grain ages, including µm) indicate that these samples cooled rapidly from some bimodal distributions, and the broader track-length temperatures >~110°C to temperatures <~50–60°C at the distributions (Appendix B of Chmielowski, 1998). The time given by the AFT age for each sample. This is data indicate that these samples resided for some period

Table 4. Fission track data sorted by physical location of samples collecteda

Nearest Central Mean Track Standard Cross Sample Fault Elevation Unit Pooled Age Length Deviation Section Number Panel Age (Ma)b (Ma)c (µm) (µm) A–A’ RMC8-7 D 5,800 ft Dhf 60.2 ± 2.6 60.2 ± 3.0 14.35 ± 0.11 1.13 A–A’ RMC7-6 C2 4,600 ft Dns? 71.2 ± 5.1 71.8 ± 14.07 13.81 ± 0.16 1.61 C–C’ RMC7-5 C2 5,700 ft MDke? 73.2 ± 5.6 73.2 ± 5.5 13.43 ± 0.11 1.09 C–C’ RMC7-1 B 4,100 ft MDku 68.7 ± 3.7 68.7 ± 3.6 13.97 ± 0.13 1.29 D–D’ RMC8-6 Z 4,500 ft MDku 60.3 ± 3.2 60.2 ± 3.8 13.86 ± 0.15 1.51 D–D’ RMC8-5 C2 4,250 ft MDke? 65.7 ± 4.5 65.7 ± 4.4 13.35 ± 0.13 1.29 D–D’ RMC8-3 B 4,000 ft MDku 71.6 ± 6.7 78.7 ± 10.3 13.92 ± 0.15 1.49 aNote changes in age as elevation changes along each section line. All analyses by Paul O’Sullivan, La Trobe University, Australia. For more detail see the full fission track report in Appendix B of Chmielowski (1998). See O’Sullivan and others (1997) for a more complete explanation. bPooled age is the best estimate of age for samples in which all grains represent a single age population. cCentral age is a better estimate where this is not the case. 26 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS of time at paleotemperatures high enough to reset some both episodes of cooling. Therefore, samples with the grain ages but not high enough to reset all grain ages. The most precise and youngest (Paleocene) AFT ages are samples with the youngest ages (RMC8-6, RMC8-7) seem juxtaposed across thrust faults over samples with older, to have been the most thoroughly overprinted (higher less precise AFT ages (fig. 4). This relationship indicates paleotemperatures) prior to the Paleocene cooling. that those samples that are now structurally highest were The mechanism of the mid-Cretaceous cooling located at greater depths (higher paleotemperatures) episode is unknown based on data from the study area before thrusting than samples now located in structurally alone. O’Sullivan and others (1997) proposed that cooling lower panels (fig. 4). This evidence supports Paleocene at this time was the result of kilometer-scale denudation activity for both fault X, which is shown elsewhere to be associated with active thrust faulting and folding within an out-of-sequence fault (Wallace and others, 1997), and the core of the Brooks Range at the end of the initial fault 3, which is the highest potentially in-sequence fault period of major shortening in the main axis of the range, and also happens to have the largest probable although they also mention the alternative possibility of displacement. unroofing by regional extension. These new results do Previous work in the central Brooks Range has not constrain the mechanism any further, but indicate that, identified 60 Ma cooling/denudation ages primarily in the whatever the mechanism, the rocks must have cooled to range-front region (Blythe and others, 1996; O’Sullivan, paleotemperatures of ~90–80°C during that time to start 1996; O’Sullivan and others, 1997), as well as locally accumulating fission tracks in the apatite grains. within internal parts of the orogen (Murphy and others, The Paleocene cooling episode, on the other hand, is 1994; O’Sullivan and others, 1998b). However, as samples most likely to be the result of more deeply buried rocks collected for these previous studies came from areas being thrust into higher structural levels within the study either south or north of the Toyuk thrust zone, those area. As shown in figure 4, the samples from panels D and studies did not find any evidence to suggest Paleocene Z gave the youngest AFT ages. In detail, the results from activity in the zone. In apparent contrast, the results from these particular samples record only the Paleocene event this study, which was the first study to sample rocks from and indicate that they must have cooled from within the fault zone itself, indicate that the Toyuk thrust temperatures >~110°C to temperatures <~50–60°C at ~60 system was indeed active—and perhaps even formed— Ma. However, the results from samples collected from the during the 60 Ma event. Furthermore, the thrust zone is lower thrust panels C-2 and B suggest that they record located ~25 km south of the range front, which suggests

Time (Ma) 150 100 50 0 0 Middle Paleocene cooling Cretaceous (?) cooling

o 60 PAZ 110 Cooling path Cooling path

Temperature ( C) lower thrust (?) (?) upper thrust panels panels 180 Maximum paleotemperatures unknown, but must have been > 110°C

Figure 10. Proposed cooling histories below ~110°C for the samples analyzed from the Toyuk thrust zone. Apatite fission track (AFT) data record two episodes of cooling, at ~100 Ma and ~60 Ma. Results from samples from the highest thrust panels record only Paleocene cooling/denudation, whereas results from samples from the lower thrust panels record both cooling events. PAZ refers to the partial annealing zone for apatite, the temperature range within which AFT ages are significantly reduced (for more explanation see Gleadow and others, 1986). See text for further details. DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 27 that structural thickening occurred within the orogenic Two end-member models presented in this study to wedge, rather than solely at its base and leading edge. account for the exposure of the various units at the Thus, the fission-track results indicate that at least some surface differ only in their interpretation of the faulting occurred in the Toyuk thrust zone at ~60 Ma in structurally highest fault, fault 3. In model one, fault 3 is a location hindward of the position of the deformation a linking fault within the duplex, and the roof has been front both during the ~60 Ma event and during earlier eroded in this area. In model two, fault 3 is the roof of the thrusting events. Deformation internal to the wedge at duplex. Neither version requires a large displacement on ~60 Ma represents a departure from a simple forward- the linking faults of the duplex. Further study of the Toyuk propagating thrust sequence as the older orogenic wedge thrust zone along strike is required to determine which continued its growth during the ~60 Ma event. Models model better represents the area. Locating both hanging- for orogenic wedges predict such apparently “out of wall and footwall cutoffs across a fault would allow the sequence” activity within wedges (Platt, 1986; Dahlen actual displacement on that fault to be determined. and Suppe, 1988; Hardy and others, 1998), but such Locating remnants of the duplex roof would allow further activity is difficult to document. control. If the rocks overlying the roof repeat the same stratigraphic section as that exposed in the duplex (as in CONCLUSIONS model two), this would support a larger displacement along the fault (fig. 11A), and the exposed length of the This study supports the conclusion that the roof across strike would provide a measure of its minimum structural geometry and evolution of the area has been displacement. However, if the rocks overlying the roof strongly influenced by its mechanical stratigraphy. The consist of younger units than exposed within the duplex, alternation of incompetent and competent units allowed it would support the smaller-displacement model (fig. the formation of detachment folds, which were then 11B). Different versions of these models can be visualized truncated by thrust faults, and ultimately led to the depending on the location and character of the facies formation of a duplex of thrust-truncated detachment change in the Kanayut Conglomerate between the thicker folds. The duplex geometry in this interpretation northern facies and the thinner southern facies (fig. 7). contrasts with other published duplex geometries (Boyer The most likely possibility is that the southernmost fault and Elliott, 1982; Mitra, 1986; Mitra and Boyer, 1986) that in the duplex was localized at the facies change and limit folding within a duplex to fault-bend folding or, additional horses formed within the northern facies in the rarely, fault-propagation folding. However, this is not the footwall of this fault. first study to suggest duplexes of thrust-truncated Regardless of the details of the structural geometry, detachment folds (Homza, 1992; Morley, 1994). This study an important conclusion is that the Toyuk thrust zone in addresses a particular example of this type of duplex, but the study area is not a single fault, but a series of thrusts such structures are widespread in the northern Brooks that, along strike, place Hunt Fork Shale over Kanayut Range (Wallace, 1993; Wallace and others, 1997) and Conglomerate. Within the study area, the southernmost likely exist in fold-and-thrust mountain ranges worldwide. fault is the fault that separates extensive Hunt Fork to the

Figure 11. Duplex roofs. (A) Older-over-younger. The stratigraphy of the duplex repeats above the roof. (B) Younger-over-older. The normal stratigraphic succession continues undisturbed above the duplex. Pattern fills do not designate bedding. 28 SHORT NOTES ON ALASKA GEOLOGY 1999 CHMIELOWSKI AND OTHERS south from extensive Kanayut to the north, and hence Lucas Heights, Australia, was supplied through an would be identified regionally as “the Toyuk thrust” Australian Institute of Nuclear Science and Engineering (fig. 2). This also is the fault most likely to separate the two grant to the La Trobe University Fission-Track Research facies of the Kanayut and to have the largest Group. We also thank Charles G. Mull and Thomas displacement. However, along strike to the east of the Moore for their reviews of this manuscript. Dalton Highway (fig. 2), Hunt Fork is separated from REERENCES Kanayut by a different fault, a deeper-level exposure of one of the smaller-displacement faults to the north. Blythe, A.E., Bird, J.M., and Omar, G.I., 1996, Deformation Thus, it should not be assumed regionally that the fault history of the central Brooks Range, Alaska: Results that forms the local boundary between Hunt Fork and from fission-track and 40Ar/39Ar analyses: Tectonics, Kanayut is necessarily either a large-displacement fault v. 15, no. 2, p. 440–455. or the fault that marks the facies change within the Boyer, S.E., and Elliott, David, 1982, Thrust systems: Kanayut. Instead, the Toyuk thrust zone should be American Association of Petroleum Geologists viewed as a complex zone of multiple thrusts that likely Bulletin, v. 66, no. 9, p. 1196–1230. formed at or near the facies change in Kanayut and may Brosgé, W.P., Reiser, H.N., Dutro, J.T., Jr., and Detterman, have significant displacement, although the amount and R.L., 1979a, Bedrock geologic map of the Philip Smith distribution of displacement likely varies along the length Mountains quadrangle, Alaska: U.S. Geological of the zone. Survey Miscellaneous Field Studies Map 897B, The apatite fission-track data from this study indicate 2 sheets, scale 1:250,000. Paleocene displacement of at least the structurally Brosgé, W.P., Reiser, H.N., Dutro, J.T., Jr., and Nilsen, T.H., highest and hindmost thrusts. Previous studies in the 1979b, Geologic map of Devonian rocks in parts of the central Brooks Range (Blythe and others, 1996; Chandler Lake and Killik River quadrangles, Alaska: O’Sullivan, 1996; O’Sullivan and others, 1997) U.S. Geological Survey Open-File Report 79-1224, documented Paleocene cooling at the range front, some 1 sheet, scale 1:200,000. 25 km to the north, but not along the Toyuk thrust. Other Chmielowski, R.M., 1998, Structural geometry and studies have documented apparent out-of-sequence evolution of the Toyuk Thrust Zone, Brooks Range, deformation at the range-front (Mull and others, 1997), Alaska. Unpublished Master’s Thesis: University of breaching thrusts within the Endicott Mountains Alaska, Fairbanks, 92 p., 2 plates, scale 1:25,000. allochthon (Wallace and others, 1997), and Paleocene Dahlen, F.A., and Suppe, John, 1988, Mechanics, growth, cooling in more internal parts of the orogen (Murphy and and erosion of mountain belts, in Clark, S.P., Jr., others, 1994; O’Sullivan and others, 1998b). These Burchfiel, B.C., and Suppe, John, eds., Processes in observations suggest that deformation has occurred continental lithospheric deformation: Geological within the orogenic wedge, at least in part during Society of America Special Paper 218, p. 161–178. Paleocene time, but they do not identify the time at which Gallagher, Kerry, 1995, Evolving temperature histories specific structures were active. 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Detailed geochronology was necessary in Grantz, Arthur, Moore, T.E., and Roeske, S.M., 1991, A-3; this case to determine that such internal deformation Gulf of Alaska to Arctic Ocean: Boulder, Colorado, occurred, and structural style or relative structural Geological Society of America Centennial Continent/ sequence were not sufficient in themselves to determine Ocean Transect no. 15, 72 p., 3 sheets, scale 1:500,000. the age of that deformation. Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.R., and Laslett, G.M., 1986, Thermal annealing of fission ACKNOWLEDGMENTS tracks in apatite; 1, A qualitative description: Chemical Geology; Isotope Geoscience Section, v. 59, This work was made possible with funding by the no. 4, p. 237–253. industry sponsors of the Tectonics and Sedimentation Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Research Group at the University of Alaska Fairbanks and Gleadow, A.J.W., and Lovering, J.F., 1989, Thermal a grant from the Geological Society of America. Support annealing of fission tracks in apatite; 4, Quantitative for the AFT sample irradiations at the HIFAR reactor at modeling techniques and extension to geological DUPLEX STRUCTURE AND PALEOCENE ACTIVITY O THE TOYUK THRUST ZONE NEAR THE DALTON HIGHWAY 29

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B9, p. 20,645–20,684. apatite; 2, A quantitative analysis: Chemical Geology, Morley, C.K., 1994, Fold-generated imbricates: examples Isotope Geoscience Section, v. 65, no. 1, p. 1–13. from the Caledonides of southern Norway: Journal of Liu, Shumin, and Dixon, J.M., 1995, Localization of duplex Structural Geology, v. 16, no. 5, p. 619–631. thrust-ramps by buckling: analog and numerical Mull, C.G., 1982, The tectonic evolution and structural modeling, Journal of Structural Geology: v. 17, no. 6, style of the Brooks Range, Alaska; An illustrated p. 875–886. summary, in Powers, R.B., ed., Geological Studies of Mayfield, C.F., Tailleur, I.L., and Ellersieck, Inyo, 1988, the Cordilleran Thrust Belt: Rocky Mountain Stratigraphy, structure, and palinspastic synthesis of Association of Geologists, Denver, Colorado, v. 1, the western Brooks Range, northwestern Alaska, in p. 1–45. 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eastern Koyukuk basin, central Brooks Range, and O’Sullivan, P.B., 1996, Late Mesozoic and Cenozoic eastcentral Arctic Slope: Alaska Division of thermotectonic evolution of the Colville Basin, Geological & Geophysical Surveys Guidebook 7, v. 1, North Slope, Alaska, in Johnsson, M.J., and Howell, p. 47–56. D.G., eds., Thermal evolution of sedimentary basins Mull, C.G., and Adams, K.E., eds., 1989, Dalton Highway, in Alaska: U.S. Geological Survey Bulletin 2142, Yukon River to Prudhoe Bay, Alaska, Bedrock p. 45–79. Geology of the Eastern Koyukuk Basin, Central O’Sullivan, P.B., and Parrish, R.R., 1995, The importance Brooks Range, and East Central Arctic Slope: Alaska of apatite composition and single-grain ages when Division of Geological & Geophysical Surveys, interpreting fission track data from plutonic rocks; a Guidebook 7, 2 vols., 327 p. case study from the Coast Ranges, British Columbia: Mull, C.G., Adams, K.E., and Dillon, J.T., 1989, Earth and Planetary Science Letters, v. 132, p. 213– Stratigraphy and structure of the Doonerak fenster 224. and Endicott Mountains allochthon, central Brooks O’Sullivan, P.B., Kohn, B.P., and Mitchell, M.M., 1998a, Range, Alaska, in in Mull, C.G., and Adams, K.E., Phanerozoic reactivation along a fundamental editors, Dalton Highway, Yukon River to Prudhoe Bay, Proterozoic crustal fault, the Darling River Lineament, Alaska: Bedrock Geology of the eastern Koyukuk Australia: constraints from apatite fission track basin, central Brooks Range, and east-central Arctic thermochronology: Earth and Planetary Science Slope: Alaska Division of Geological & Geophysical Letters, v. 164, no. 3–4, p. 451–465. 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Suppe, John, 1983, Geometry and kinematics of fault-bend propagation folds; Examples from the Brooks Range: folding: American Journal of Science, v. 283, no. 7, Geological Society of America Abstracts with p. 684–721. programs, v. 28, no. 7, p. A-239. Suppe, John, and Medwedeff, D.A., 1990, Geometry and Wallace, W.K., and Homza, T.X., 1997, Differences kinematics of fault-propagation folding, in Jordan, between fault-propagation folds and detachment Peter, Noack, Thomas, Schmid, Stefan, and Bernoulli, folds and their subsurface implications: Annual Daniel, eds., The Hans Laubscher volume: Eclogae Meeting Abstracts, American Association of Geologicae Helvetiae, v. 83, no. 3, p. 409–454. Petroleum Geologists and Society of Economic Wallace, W.K., 1993, Detachment folds and a passive-roof Paleontologists and Mineralogists, v. 6, p. A122. duplex: Examples from the northeastern Brooks Wallace, W.K., Moore, T.E., and Plafker, George, 1997, Range, Alaska, in Solie, D.N., and Tannian, Fran, eds., Multistory duplexes with forward dipping roofs, Short Notes on Alaskan Geology 1993: Alaska north central Brooks Range Alaska, in Plafker, Division of Geological & Geophysical Surveys George, and Mooney, W.D., eds., The Trans-Alaska Professional Report 113, p. 81–99. Crustal Transect (TACT) across Arctic Alaska: Wallace, W.K., and Homza, T.X., 1996, Thrust-truncated Journal of Geophysical Research, v. 102, no. B9, detachment folds and their distinction from fault- p. 20,773–20,796.

BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS O THE NORTHEASTERN NORTH SLOPE, ALASKA Catherine L. Hanks,1 Melissa Parker,1 and Emily B. Jemison1

ABSTRACT

Significant variations exist in the orientation of present-day horizontal in situ stresses in the North Slope of northeastern Alaska. In situ horizontal stress orientations based on borehole breakouts in 30 wells north and west of the Arctic National Wildlife Refuge (ANWR) exhibit two different patterns. Maximum horizontal stress orientations (Shmax) generally are perpendicular to the deformation front in wells immediately north of the northeastern Brooks Range fold-and-thrust belt. Shmax orientations range from north-northeast trending offshore of northern ANWR to northwest trending near the Dalton Highway. In situ stresses in this region are probably due to active compression related to the fold-and-thrust belt. In contrast, borehole breakout orientations in wells on or near the Barrow arch that are more distal to the thrust front are more variable and commonly have bimodal orientations. These data can be interpreted to represent Shmax orientations that are northwest and northeast trending. These orientations may reflect in situ stresses resulting from either extension along the northern Alaska continental margin, complex interactions between extension along the Barrow arch and stresses generated by thrusting from the south, or local stress perturbations on the Barrow arch associated with complex local fault patterns.

INTRODUCTION

In situ stresses can play a significant role in hydro- orientation of present-day in situ horizontal stresses in carbon exploration and production. Low differential the subsurface of northeastern Alaska; and (2) develop stresses in the foreland basin of a fold-and-thrust belt can a predictive model or models that might explain the be an important factor in developing and maintaining an observed regional stress regime. open fracture network in rocks forward of the growing thrust belt (for example, Lorenz and others, 1991). These MEASURING IN SITU STRESSES IN fractures in turn can be influential in controlling fluid SEDIMENTARY BASINS movement through the foreland basin and can enhance reservoir characteristics in potential reservoir rocks. In A variety of methods exist to measure the orientation situ stresses also influence fluid flow in producing and magnitude of in situ stresses, with varying degrees reservoirs by controlling the direction of fracture and fault of reliability. The technique used often is governed by permeability. Determining the direction of the in situ what data are available. The two most common and widely stress can therefore provide a predictive tool for used techniques to measure the orientation of in situ evaluating both the fluid movement within a basin and the stresses are borehole breakouts and earthquake focal development of fracture porosity and permeability in mechanisms (for example, Zoback, 1992; Bell and others, potential and producing reservoir rocks, as well as aid in 1995), with the former most widely used in sedimentary the development of production strategies. basins. Borehole breakouts were first used as an indicator The importance of understanding the distribution of of in situ horizontal stress by Bell and Gough (1979) and in situ stresses on the North Slope of Alaska has been Hottman and others (1979), and have subsequently been highlighted by recent work on the Carboniferous used in both local and regional stress studies around the reservoir interval of the Lisburne field of the North Slope world (for example, Zoback and Zoback, 1991). Breakouts of Alaska, which suggests that production from the field are intervals in wells that have undergone preferential is controlled in part by fractures held open by present-day caving or spalling due to differential horizontal stress, in situ horizontal stresses (Hanks and others, 1997). such that the diameter of the borehole becomes Production strategies therefore would be greatly aided by elongated parallel to the least horizontal stress. The shape a better understanding of the in situ stress regime of the and orientation of the borehole elongation can be North Slope. However, the present-day regional in situ measured with a variety of downhole tools, the most stress distribution and orientation in northern Alaska is common being the four-arm dipmeter tool. These poorly constrained and/or documented. The objective of measurements are routinely done on most exploratory this study is to: (1) document the regional distribution and wells; consequently data for measuring in situ horizontal

1Department of Geology & Geophysics, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, Alaska 99775-5780. Email for Catherine Hanks: [email protected]

33 34 SHORT NOTES ON ALASKA GEOLOGY 1999 HANKS AND OTHERS stress are widely available. The technique for deriving the west and the Canadian Cordillera to the south (Moore and orientation of in situ stress orientations from borehole others, 1985). The nature of the boundaries of this breakouts is summarized in several publications (for deforming belt remain uncertain; only two focal example, Bell and Gough, 1979; Bell, 1990; and Bell and mechanisms are available (Estabrook and others, 1988; others, 1995). Fujita and others, 1983, 1990) from the central part of the Beaufort Sea, both suggesting strike-slip mechanisms. REGIONAL SETTING However, the orientations of the faults on which these earthquakes occurred are not known. The location of this The Brooks Range is the northernmost part of the seismically active zone corresponds to the limit of folded Rocky Mountain fold-and-thrust belt (fig. 1). The majority and thrust-faulted Colville basin sediments observed in of shortening in the fold-and-thrust belt occurred in Late the subsurface (fig. 1). The young and probably active Jurassic to Early Cretaceous time when a wide, south- nature of this thrust front is also supported by young facing late Paleozoic to early Mesozoic passive con- uplift ages in the range front of the northeastern Brooks tinental margin collapsed in response to the collision of Range and in related parts of the central Brooks Range an intraoceanic arc (Mayfield and others, 1988; Moore (for example, O’Sullivan and others, 1993 and 1997). and others, 1994). The Colville basin formed in advance The driving mechanism for this deformation is of, and was filled with sediment shed from, the growing unclear. Directional indicators of the regional in situ fold-and-thrust belt (Mull, 1985; Molenaar and others, stress pattern, such as borehole breakouts and earth- 1987; Bird and Molenaar, 1987). quake focal mechanisms, have been summarized for all of Shortly after the main phase of compressional Alaska by Estabrook and Jacob (1991). The overall collapse of the continental margin, rifting led to formation pattern is one of north-south oriented compressive of the oceanic Canada basin to the north (present geo- stresses in the southern part of Alaska, orthogonal to the graphic coordinates) in Early Cretaceous time (Grantz and convergent Pacific/North American plate boundary. The May, 1983; Moore and others, 1994). This continental orientation of the maximum horizontal in situ stress breakup resulted in separation of the Brooks Range and diverges to the north, becoming progressively more east- North Slope from the continent to the north and formation west towards northwestern Alaska, and rotating to a more of the present northern continental margin of Alaska. northeasterly orientation in northwestern Canada. This new continental margin is at an oblique angle to the Estabrook and Jacob (1991) attributed this fanning of the Brooks Range orogen (fig. 1). in situ stress regime in northern Alaska to the ‘indentor Subsequent shortening has progressed to different effect’ caused by collision of the Yakutat block with extents along the strike of the range and its foreland basin, southern Alaska. and consequently has involved different parts of both the earlier south-facing passive continental margin and RESULTS O THIS STUDY the new north-facing continental margin. In the western PROCEDURE Brooks Range, the thrust front is far south of the present northern continental margin, the Colville basin is wide The four-arm dipmeter tool is one of the oldest and and, to the north, undeformed by thrust deformation; in most widely used downhole tools for measuring the the northeastern Brooks Range, the thrust front has orientation of planar features in the rocks as well as the overridden the later-rifted continental margin, resulting in shape and orientation of the borehole. The tool consists a very narrow and deformed foreland basin. of electrodes located on four orthogonal pads pressed against the borehole wall by hydraulically activated STATE O KNOWLEDGE REGARDING IN SITU caliper arms. The electrodes measure the resistivity of the STRESSES IN NORTHERN ALASKA rock adjacent to the borehole as the tool is drawn up the hole. Besides the resistivity traces themselves, the Although the majority of shortening associated with uncomputed log shows the orientation of one of the pads the Brooks Range is Mesozoic in age, the pattern of active relative to true north and the amount of extension or seismicity in northern Alaska and northwestern Canada contraction of the pads as the tool rotates up the hole. indicates that the northeastern part of the Brooks Range This provides a means of locating and measuring the and Colville basin is seismically active (fig. 2; Basham orientation of a borehole elongation. If a borehole and others, 1977; Biswas and Gedney, 1979). This elongation is encountered as the tool rotates up the hole, contrasts with the remainder of northern Alaska, which one set of pads will extend into the wider part of the is seismically quiet (Estabrook and others, 1988). Active borehole, causing the tool to stop rotating as it is drawn seismicity outlines the northeastern Brooks Range in up the hole. Alaska and Canada, and defines an arcuate thrust front Wide zones in the borehole can be due to a variety of that connects the older part of the Brooks Range to the different factors. The four most common borehole BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS, NORTHEASTERN NORTH SLOPE 35 ied in table 1. Regional map of northern Alaska showing major structural features and locations of wells used in this study. Wells are identif are Wells and locations of wells used in this study. Regional map of northern Alaska showing major structural features Figure 1. 36 SHORT NOTES ON ALASKA GEOLOGY 1999 HANKS AND OTHERS

Figure 2. Map showing the regional stress indicators and pattern of active seismicity of northern Alaska (1968– 1978) and northwestern Canada (1962–1974). This map also illustrates the paucity of stress orientation data from northern Alaska. Regional stress indicators in northeastern Alaska consist of two earthquake focal mechanisms and breakouts in ~12 wells, with the majority of the wells clustered in one area on the Barrow arch. There is no information on the orientation of in situ stresses from the central or western part of the Colville basin or North Slope. Modified from Basham and others (1977), Biswas and Gedney (1979), and Estabrook and Jacob (1991). geometries include in-gauge hole, breakout, washout, and key seating (fig. 3; Plumb and Hickman, 1985; Bell and others, 1995). In order for a wide zone to be identified as a ‘breakout,’ it must meet certain criteria, including: • the tool rotation stops in the zone of the breakout; • the difference between the two sets of pads is greater than 0.6 cm (0.24 inches); • the smaller of the two caliper readings is close to the bit size, or if greater than the bit size, should exhibit less variation than the larger caliper reading; • the vertical extent of the zone of borehole elongation is greater than 30 cm (11.8 inches); and • the direction of the zone of elongation should not coincide with the azimuth of the high side of the borehole when the hole deviates from vertical (that is, keyseating or breakouts due to vertical stresses on a deviated hole [John Lorenz, written commun., 1999]).

Figure 3 (right). Four common borehole geometries and their corresponding caliper separations. Modified from Plumb and Hickman (1985). BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS, NORTHEASTERN NORTH SLOPE 37

In this study, determination that conditions 1, 2, 3, and using the World Stress Map (WSM) quality classification 4 were met was by visual inspection while analyzing the criteria (table 2; Zoback, 1992). well logs. Condition 3 guarantees that a washout will not be mistaken for a breakout. Condition 5 precludes either RESULTS a keyseat or a breakout due to vertical stresses from being Four-arm uncomputed dipmeter logs from over mistaken as a breakout due to in situ horizontal stresses, 60 wells were examined for borehole breakouts. Only and was assessed both by inspection and by plotting the 30 wells had measurable breakouts (fig. 1, table 1). Of orientation of the breakouts versus the trajectory in these, 16 wells had a sufficient number and consistency individual wells. If a borehole elongation was parallel of breakouts that the wells could be classified as A, B, or (±10°) to the azimuth of the high side of the borehole, it C quality according to the WSM criteria (table 2). The was discarded. Wells with breakouts then were classified breakout orientation in the A, B, and C quality wells could Table 1. Wells used in this study. Table includes the number of breakouts observed in each well, the total length of breakouts, the World Stress Map (WSM) classification (table 2) and whether the breakout distribution was bimodal or unimodal.

Map No. of Total Breakout WSM no. Well Name Breakouts Length(m) Orientation St. Dev Class 1 ARCO Brontosaurus 1 9 2 0 E 2 ARCO Ocs 0267(Fireweed) 1 1 7 30 0 E 3 ARCO W Sak 25590 15 1 41 63 40.9 D 4 ARCO Beechy Pt 1 9 78 173 79 C 5 ARCO Kru W Sak 26 1 88 62.5 0 D 6 ARCO Se Eileen State 1 1 16 153 0 D 7 ARCO Highland State 1 49 153 0 D 8 ARCO Prudhoe Bay Unit Term A 9 571 336 23.3 B 9 BP Put River St 1 2 170 151 11.2 D 10 ARCO Gull Island State 1 8 205 325 109.9 (BI) B 47 11 ARCO E Bay St 1 3 119 41.7 9.2 D 12 ARCO Delta St 2 3 170 320 18.5 D 13 ARCO Delta St 1 3 88 173 16.0 D 14 ARCO Lake St 1 5 172 42 61.7 C 15 BP Sag Delta 31-10-16 2 71 134 0 D 16 Mobil Kadler 15-9-16 5 108 146 37.7 C 17 Chevron Karluk 1 2 137 78.7 8.0 D 18 ARCO West Mikkelsen Unit 2 6 164 325 233.3 (BI) B 40 19 ARCO W Mikkelsen St 1 8 491 62 39.7 (BI) B 160 20 Exxon Alaska St A 1 6 646 69 18.7 B 21 Unocal E De K Leffingwell 1 9 900 177 24.0 B 22 Texaco West Kavik 1 5 683 152 7.1 C 23 Pan Am Kavik 1 1 58 73 0 D 24 ARCO Kavik Unit 2 9 242 313 65.1 B 25 ARCO Gyr 1 7 188 57 167.1 (BI) B 325 26 Exxon Canning River Unit B 1 8 326 117 5.6 B 27 ARCO Nora Federal 1 11 798 133 4.8 A 28 Tenneco Ocs 0943(Aurora) 1 13 345 181 8.9 A 29 Amoco Ocs 0917(Belcher) 1 2 50 83 2.5 D 30 ARCO Big Bend 1 8 281 31 82.9 B 38 SHORT NOTES ON ALASKA GEOLOGY 1999 HANKS AND OTHERS

Table 2. World Stress Map (WSM) classification criteria between breakouts and depth, however. Breakout for borehole breakouts used to rank the quality of orientation becomes significantly more consistent at data in this study. From Zoback (1992). depths below 4,000 ft (subsea) as seen in wells 24, 25, 8, and 20 (fig. 6), and is probably a function of the A ≥10 distinct breakouts in a single well with s.d. ≤12° magnitude of Shmax versus the amount of overburden. and/or combined breakout length >300 m This suggests that Shmax may not be reliably derived from shallow (<4,000 ft deep) breakouts. Average of breakouts in ≥2 adjacent wells with ≤ combined length >300 m and s.d. 12° THRUST RONT WELLS ≥ B 6 distinct breakout zones in a single well with s.d. In wells drilled adjacent to the thrust front, Shmax is ≤20° and/or combined length >100 m generally oriented north-northwest, perpendicular to the active thrust front of the northeastern Brooks Range C ≥4 distinct breakouts with s.d. <25° and/or combined (fig. 5). Wells that exhibit this stress pattern include nos. length >30 m 22, 24, 26, 27, 28 (table 1, figs. 1, 3, 4, 6). Out of the nine wells examined in this study that were located near the D <4 consistently oriented breakouts or <30 m thrust front, seven (78 percent) were ranked as having combined length in a single well either A, B, or C quality breakouts (table 1). The only wells Breakouts in a single well with s.d. ≥25° in the study that were ranked as ‘A’ quality, nos. 27 (ARCO Nora Federal #1) and 28 (Tenneco Aurora #1) E Wells in which no reliable breakouts detected. both fall into this group. Breakout orientations in the thrust front wells are consistent regardless of Extreme scatter of orientations, no significant mean stratigraphic interval and at all depths below ~4,000 ft determined (>40°) (subsea). This all implies (although doesn’t necessarily prove) that the in situ stress signal in this area is fairly strong. The close correspondence between the direction then be used with some degree of confidence to evaluate of maximum in situ stresses and the active thrust front the in situ stress orientation at that location. suggests that the stress pattern in this part of north- Rose diagrams of the breakout orientations in these eastern Alaska is strongly influenced by the thrust front. 16 wells are shown in figure 4. The orientation of the Two wells west of the main cluster of range front inferred maximum in situ horizontal stress (Shmax) direction wells, nos. 25 (ARCO Gyr #1) and 30 (ARCO Big Bend #1), (corresponding to the perpendicular to the orientation of are immediately adjacent to the thrust front and have the long axis of the breakout) is also illustrated on the strong breakout signals, but exhibit anomalous breakout regional map in figure 5. The orientation of the short axes directions. The Shmax orientation inferred from breakout of breakouts in each well versus the depth of each orientations in ARCO Big Bend #1 trends northeast. This breakout and the stratigraphic unit (where known) is well is located where the northern limit of active seismicity shown in figure 6. in the northeastern Brooks Range trends southwest into DISCUSSION the central Brooks Range (fig. 2). The northeast trend of Shmax in this region may reflect a change in the in situ Borehole breakout data acquired during this study stress regime from north-northwest-oriented suggest that there are at least two local in situ stress compression at the deformation front of the northeastern patterns in the eastern part of the North Slope of Alaska. Brooks Range to a more transpressive regime at the Shmax in wells adjacent to the mappable deformation front deformation front of the central Brooks Range. is generally oriented north-northwest, perpendicular to The second anomalous range-front well, ARCO Gyr the thrust front. In contrast, Shmax in wells along the #1, shows a bimodal breakout distribution (fig. 4). This Barrow arch have both north-northwest and northeast breakout distribution may reflect low overall in situ stress orientations, sometimes in one well. The following section magnitudes in the vicinity of that well and/or the will discuss these two stress patterns in more detail. influence of local structures. However, if the breakouts There is no clear relationship between breakouts and from depths <4,000 ft (fig. 6) in this well are assumed to gross stratigraphic interval (fig. 6). Breakouts occur in be anomalous and eliminated, the inferred Shmax direction both the Cretaceous and younger clastic Brookian becomes unimodal and in the same general direction as sequence and the Mississippian to Lower Cretaceous ARCO Big Bend #1. This would imply that the change in mixed clastic and carbonate rocks of the Ellesmerian orientation of Shmax seen in ARCO Big Bend #1 may be sequence. There does appear to be a relationship regional in scope. BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS, NORTHEASTERN NORTH SLOPE 39 ). hmax horizontal stress (S in situ Orientation of the short axes of the borehole in breakout intervals in wells classified as A, B, or C (World in breakout axes of the borehole Orientation of the short Stress Map [WSM] classification) on table 2. The orientation of the short axis borehole in a breakout interval can be interpreted to parallel the maximum Figure 4. 40 SHORT NOTES ON ALASKA GEOLOGY 1999 HANKS AND OTHERS

BARROW ARCH WELLS passive margins, and could be interpreted in a variety of ways: A significantly different stress pattern is localized on • Faults could be influencing the in situ stress and adjacent to the eastern end of the Barrow arch (figs. patterns in nearby wells (for example, Mount and 4 and 5). Of the 21 wells examined along the Barrow arch Suppe, 1987; Evans, 1989; Castillo and Zoback, in this study, only nine (43 percent) are ranked as A, B, or 1994), causing permutations in a regime where the C class (table 1). The relatively low percentage of wells overall horizontal maximum stress is oriented north- with a strong to moderately strong breakout signal could northwest; be interpreted to reflect a relatively low in situ horizontal • The distance from the active thrust front may have stress signal. resulted in a lower value of Shmax, causing the The breakout pattern seen in this area is exemplified maximum and minimum in situ stresses in this area by wells 14, 18, 19, and 20 (figs. 4, 5, and 6). At first glance, to be close to one another in value. This could result Shmax orientations in these wells show a bimodal distribu- in borehole breakouts parallel to both directions (for tion. However, one well (#20, Exxon Alaska State 1) example, Castillo and Zoback, 1994), both in becomes unimodal with depth (albeit still an anomalous individual wells, and between nearby wells; orientation), and a second (#14, ARCO Lake State 1) has • The Barrow arch could represent a totally different, few breakouts below 5,000 ft, leaving only two wells (16 possibly extensional, stress regime than the com- and 19) with true bimodal breakout distributions. pressional stress regime further to the south adjacent Other wells located nearby along the axis of the to the thrust front. In an extensional regime the Barrow arch, including nos. 4, 8, 10, 16, and possibly 21, maximum stress is vertical, and the minimum and are unimodal. Most, but not all, have breakout orienta- intermediate stresses are both horizontal, leading to tions similar to those seen in the deformation front wells a situation in which the horizontal stresses could be further south (fig. 4). close in value and breakouts could occur in both This mixing of bimodal and unimodal breakout orientations; and patterns in wells along the Barrow arch is not typical of • The pattern of in situ stresses seen on the Barrow arch is a result of compressive stresses set up by the thrust front interacting with the basement high.

Figure 6 (right). The orientation of the short dimension of breakouts in each well versus the depth of each breakout and the stratigraphic unit (where known). Shmax is inferred to be parallel to the short dimension of the breakout. Brookian sequence rocks consist of Lower Cretaceous and Figure 5 (above). Enlarged eastern portion of northern Alaska map younger clastic rocks; Ellesmerian shown in figure 1. This map of northeastern Alaska shows sequence rocks consist of Mississip- orientations of Shmax as interpreted from the borehole breakout pian through Jurassic clastic and data in wells classified as A, B, or C (fig. 3; table 1). Shmax carbonate rocks. Note that limited direction in wells with a single consistent breakout orientation vertical distribution of breakouts in is shown in heavy lines; thin black lines indicate the orientation some wells is probably due to limited of Shmax in wells with a consistent bimodal breakout data availability. Stratigraphic distribution. Wells are numbered and identified as in table 1. information was not available for all wells. BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS, NORTHEASTERN NORTH SLOPE 41 42 SHORT NOTES ON ALASKA GEOLOGY 1999 HANKS AND OTHERS

CONCLUSIONS Bell, J.S., and Gough, D.I., 1979, Northeast-southwest compressive stress in Alberta: Evidence from oil wells: Borehole breakouts in wells from the eastern North Earth and Planetary Science Letters, v. 45, no. 2, Slope of Alaska can be interpreted to reflect the present- p. 475–482. day in situ horizontal stress regime of northeastern Bell, J.S., Lorenz, J.C., Aguilera, R., and McLellan, P.J., Alaska. Borehole breakouts in this region occur 1995, Fractured reservoirs—Their recognition, throughout the Late Paleozoic and younger section. evaluation and production; course notes: Canadian Breakout orientations are most consistent, however, at Society of Petroleum Geologists & Canadian Well depths >4,000 ft (subsea). Logging Society. There are two main patterns in the orientation of Bird, K.J., and Molenaar, C.M., 1987, Stratigraphy: in Bird, present-day Shmax in the subsurface of northeastern K.J., and Magoon, L.B., eds., Petroleum geology of Alaska. Shmax orientations generally are perpendicular to the northern part of the Arctic National Wildlife the deformation front in wells immediately north of the Refuge, northeastern Alaska: U.S. Geological Survey northeastern Brooks Range fold-and-thrust belt, Bulletin 1778, p. 37–59. suggesting that Shmax in this region is probably due to Biswas, N.N., and Gedney, L., 1979, Seismotectonic active compression related to the fold-and-thrust belt. In studies of northern and western Alaska: National contrast, wells on or near the Barrow arch are more distal Oceanic and Atmospheric Administration Report, to the thrust front. Inferred Shmax orientations in this 50 p. region are more variable, sometimes bimodal, in distribu- Castillo, D.A., and Zoback, M.D., 1994, Systematic tion, and generally are northwest and northeast trending. variations in stress state in the southern San Joaquin The variable orientations in Shmax along the Barrow Valley; inferences based on well-bore data and arch may be due to either (1) extension along the northern contemporary seismicity: American Association of Alaska continental margin, (2) complex interactions Petroleum Geologists Bulletin, v. 78, no. 8, p. 1,257– between extension along the Barrow arch and stresses 1,275. generated by thrusting from the south, (3) local stress Estabrook, C.H., and Jacob, K.H., 1991, Stress indicators perturbations on the Barrow arch associated with in Alaska, in Slemmons, D.B., Engdahl, E.R., Zoback, complex local fault patterns, or (4) a combination of all four M.D., and Blackwell, D.D., eds., Neotectonics of factors. North America; The Geology of North America, Decade Map Vol. 1: Boulder, Colorado, Geological ACKNOWLEDGMENTS Society of America, p. 387–399. Estabrook, C.H., Stone, D.B., and Davies, J.N., 1988, We would like to thank the National Science Seismotectonics of northern Alaska: Journal of Foundation for their support of the Geophysical Institute Geophysical Research, v. 93, no. B10, p. 12,026–12,040. Summer Intern Program, which provided support for M. Evans, K.F., 1989, Appalachian stress study 3, Regional Parker and E. Jemison. Logs were provided by ARCO scale stress variations and their relation to structure Alaska, Inc., BP Exploration, and Chevron. Additional and contemporary tectonics: Journal of Geophysical support was provided by industry sponsors of the Research, v. 94, no. 12B, p. 17,619–17,645. Tectonics and Sedimentation Research Group, including Fujita, Kazuya, Cook, D.B., and Coley, M.J., 1983, ARCO Alaska, Inc., BP Exploration, Chevron, Phillips, Tectonics of the western Beaufort and Chukchi Seas: Petrofina, and Union Texas. We also wish to thank the American Geophysical Union, 1983 spring meeting, reviewers who kindly provided their comments on this EOS Transactions, American Geophysical Union, paper, including Wes Wallace, John Lorenz, and Richard v. 64, no. 18, p. 263. Fox. Fujita, Kazuya, Cook, D.B., Hasegawa, H.S., Forsyth, David, and Wetmiller, R.J., 1990, Seismicity and focal REERENCES mechanisms of the Arctic region and the North American plate boundary in Asia, in Grantz, Arthur, Basham, P.W., Forsyth, D.A., and Wetmiller, R.J., 1977, Johnson, L., and Sweeney, J.F., eds., The Arctic The seismicity of northern Canada: Canadian Journal Ocean region, The Geology of North America, v. L, of Earth Sciences, v. 14, no. 7, p. 1,646–1,667. p. 79–100. Bell, J.S., 1990, Investigating stress regimes in Grantz, Arthur, and May, S.D., 1983, Rifting history and sedimentary basins using information from oil structural development of the continental margin industry wireline logs and drilling records, in Hurst, north of Alaska, in Watkins, J.S., and Drake, C.L., A. Lovell, M.A., and Morton, A.C., eds., Geological eds., Studies in continental margin geology: applications of wireline logs: Geological Society American Association of Petroleum Geologists Special Publications, v. 48, p. 305–325. Memoir 34, p. 77–100. BOREHOLE BREAKOUTS AND IMPLICATIONS OR REGIONAL IN SITU STRESS PATTERNS, NORTHEASTERN NORTH SLOPE 43

Hanks, C.L., Lorenz, John, Teufel, L.W., and Krumhardt, Mount , V.S., and Suppe, John, 1987, State of stress near A.P., 1997, Lithologic and structural controls on the San Andreas fault; implications for wrench natural fracture distribution and behavior within the tectonics: Geology, v. 15, no. 12, pp. 1,143–1,146. Lisburne Group, northeastern Brooks Range and Mull, C.G., 1985, Cretaceous tectonics, depositional North Slope subsurface, Alaska: American cycles, and the Nanushuk Group, Brooks Range and Association of Petroleum Geologists Bulletin, v. 81, Arctic Slope, Alaska, in Huffman, A.C., Jr., ed., no. 10, p. 1,700–1,720. Geology of the Nanushuk Group and related rocks, Hottman, C.E., Smith, J.H., and Purcell, W.R., 1979, North Slope, Alaska: U. S. Geological Survey Bulletin Relationship among earth stresses, pore pressure and 1614, p. 7–36. drilling problems, offshore Gulf of Alaska: Journal of O’Sullivan, P.B., Green, P.F., Bergman, S.C., Decker, John, Petroleum Technology, v. 31, no. 11, p. 1,477–1,484. Duddy, I.R., Gleadow, J.W., and Turner, D.L., 1993, Lorenz, J.C., Teufel, L.W., and Warpinski, N.R., 1991, Multiple phases of Tertiary uplift and erosion in Regional fractures 1: A mechanism for the formation the Arctic National Wildlife Refuge, Alaska, of regional fractures at depth in flay-lying reservoirs: revealed by apatite fission track analysis: American American Association of Petroleum Geologists Association of Petroleum Geologists Bulletin, v. 77, Bulletin, v. 75, no. 11, p. 1,714–1,737. no. 3, p. 359–385. Mayfield, C.F., Tailleur, I.L., and Ellersieck, Inyo, 1988, O’Sullivan, P.B., Murphy, J.M., and Blythe, A.E., 1997, Stratigraphy, structure, and palinspastic synthesis of Late Mesozoic and Cenozoic thermotectonic the western Brooks Range, northwestern Alaska, in evolution of the central Brooks Range and adjacent Gryc, George ed., Geology and exploration of the North Slope foreland basin, Alaska; including fission National Petroleum Reserve in Alaska, 1974 to 1982: track results from the Trans-Alaska Crustal Transect U.S. Geological Survey Professional Paper 1399, (TACT): Journal of Geophysical Research, v. 102, p. 143–186. no. B9, pp. 20,821–20,845. Molenaar, C.M., Bird, K.J., and Kirk, A.R., 1987, Plumb, R.A., and Hickman, S.H., 1985, Stress-induced Cretaceous and Tertiary stratigraphy of northeastern borehole elongation; a comparison between the four- Alaska, in Tailleur, I.L., and Weimer, Paul, eds., arm dipmeter and the borehole televiewer in the Alaskan North Slope geology, field trip guidebook: Auburn geothermal well: Journal of Geophysical Pacific section, SEPM and Alaska Geological Research, v. 90, B7, p. 5,513–5,521. Society, Book 50, p. 513–528. Zoback, M.L., 1992, First- and second-order patterns of Moore, T.E., Wallace, W. K., Bird, K.J., Karl, S.M., Mull, stress in the lithosphere; The World Stress Map C.G., and Dillon, J.T., 1994, Geology of northern Project, in Zoback, M.L., leader, The World Stress Alaska, in Plafker, George, and Berg, H.C., eds., The Map Project: Journal of Geophysical Research, v. 97, Geology of Alaska, the Geology of North America: no. B8, p. 11,703–11,728. Boulder, Colorado, Geological Society of America, Zoback, M.D., and Zoback, M.L., 1991, Tectonic stress v. G-1, p. 49–140. field of North America and relative plate motions, in Moore, T.E., Whitney, J.W., and Wallace, W.K., 1985, Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Cenozoic north-vergent tectonism in northeastern Blackwell, D.D., eds., Neotectonics of North America, Alaska; indentor tectonics in Alaska?: EOS The Geology of North America, Decade Map Vol. 1: Transactions, American Geophysical Union, v. 66, Boulder, Colorado, Geological Society of America, no. 46, p. 862. p. 339–366. MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN), NORTHWESTERN DELONG MOUNTAINS, WESTERN ARCTIC SLOPE, ALASKA David L. LePain,1 Karen E. Adams,2 and Charles G. Mull1

INTRODUCTION

The State of Alaska Division of Geological & work in the area, we conclude with some speculative Geophysical Surveys (DGGS) has conducted geological suggestions regarding the Neocomian paleogeography of field studies in the northwestern DeLong Mountains the western Arctic. since 1993 as part of an assessment of the hydrocarbon potential of the Colville Basin in the western Arctic PREVIOUS WORK Slope (fig. 1). This assessment has included detailed geologic mapping at a scale of 1:63,360, analysis of field Crane and Wiggins (1976) introduced the informal samples for organic geochemistry and thermal history, name Tingmerkpuk sandstone for Lower Cretaceous biostratigraphy, petrography, apatite fission track cooling quartzose rocks exposed in the Brooks Range foothills of and uplift history, and stratigraphic studies of selected the DeLong Mountains. The unit was dated as stratigraphic units such as the Tingmerkpuk sandstone. Valanginian age, based on microfossils. One of the best This report summarizes stratigraphic studies carried exposures of these rocks is on the north side of out during the 1998 field season on exposures of the Tingmerkpuk Mountain, which serves as the type section Tingmerkpuk sandstone on the northwest side of for the Tingmerkpuk sandstone (fig. 2). Tingmerkpuk Mountain, and at an outcrop near the head Detailed mapping of the Tingmerkpuk outcrop belt of Eagle Creek (fig. 1). The purpose of this work was to at a scale of 1:63,360 by Mull, Reifenstuhl, and Harris (in conduct a scintillometer survey through the Tingmerk- preparation) has documented two parallel facies belts puk Mountain section previously measured by R.K. within the Tingmerkpuk. The northern facies belt, which Crowder and K.E. Adams in 1993 (Crowder and others, contains the type section on Tingmerkpuk Mountain, 1994), and T.N. Nilsen, T.E. Moore, and M.D. Myers in consists of ~130 m of interbedded fine- to very fine- 1996 (Nilsen, 1996). The scintillometer survey (not grained quartzose sandstone and black clay shale. The included here) was conducted to provide a pseudo- southern facies belt, which is superimposed over the gamma ray log that could be used for correlating the northern belt by thrust faulting, consists of a generally outcrop stratigraphy with subsurface stratigraphy to the finer grained, more thinly bedded, section of quartzose north and northeast. The section was remeasured so that sandstone, siltstone, and interbedded clay shale. Owing the scintillometer readings could be accurately tied to the to generally poor exposures of this muddier facies of the measured section. Owing to the disparate interpretations Tingmerkpuk, the thickness of the southern facies belt of the depositional environments presented by Crowder has not been measured but is estimated to be about the and others (1994) and Nilsen (1996), the section was same thickness as the unit in the northern facies belt. redescribed in detail while conducting the scintillometer Mickey and others (1995) assigned a Valanginian age survey. to the base of the Tingmerkpuk sandstone based on the Previous work on the Tingmerkpuk sandstone along macrofossil Buchia sublaevis and an abundant paly- with our description and interpretation of the unit are nomorph flora, and a Hauterivian to Barremian age to the summarized in the following sections of this report. Due top of the Tingmerkpuk based on palynomorphs. This to limited outcrop and subsurface control and the indicates that the Lower Cretaceous unconformity (LCu) complex fold and thrust history of the area, it is difficult horizon is contained somewhere within the Tingmerkpuk to reconstruct Neocomian paleogeography of the sandstone. The underlying Kingak Shale ranges up into western Arctic. At its present location the Tingmerkpuk the Valanginian (Mickey and others, 1995). sandstone is nearly 350 km south of the Barrow arch, Petrographic studies by Reifenstuhl (1997) show that placing it several hundred kilometers south of other the Tingmerkpuk sandstones can be characterized as known Neocomian shelf sandstones of similar compositionally mature very fine- to medium-grained provenance. In an effort to help focus future stratigraphic quartz arenite.

1Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks Alaska 99709-3645. Email for David L. LePain: [email protected] 2U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California 94025

45 46 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS o 161 Kbl Kto Anticline Driftwood Kti MzPzs Kbl Quadrangle ALASKA GULF OF NPRA Knl Misheguk Mountain ARCTIC OCEAN Kbl Illingnorak Ridge o 162 Kti DeLong Mountains Quadrangle BERING SEA Kto Knl Knu Tingmerkpuk Mtn. Kbl Kbl Km Mi Kti 10 16 o KJk Knl 163 Kto 5 8 Kbl Knu Coke Basin 0 0 Knu Knl Knl Quadrangle KJk DeLong Mountains Knu Knl o Knu 164 Kto KJk Nanushuk Group (Albian - Cenomanian) deltaic deposits Torok Shale (Albian) pro delta shale Lower Brookian (Aptian - Barremian?) shale and graywacke turbidites Tingmerkpuk sandstone and Kingak Shale (Neocomian - Upper Jurassic) Mesozoic and Paleozoic sedimentary rocks undifferentiated Kto Knl Kbl KJk Knu KJk Kto Kbl Knu Kti MzPzs Knl Chukchi Sea Kbl Creek tis The o 165 o 69 o o 68 45’ 68 30’ MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)47

Geochemical studies of Triassic to Cretaceous shales Barrow arch and a deeper water basin toward the south; in the northwestern DeLong Mountains indicate that the sandstones were assumed to be derived from a northern clay shales interbedded with the Tingmerkpuk source terrane). In this model, the Tingmerkpuk sandstones, the underlying Kingak Shale, and the sandstone is interpreted as a deeper water (lowstand fan) overlying Lower Brookian shale contain up to 1.8 deposit that is coeval with shallow-marine Neocomian percent total organic carbon (TOC) (Mull, 1995). sandstones along the Barrow arch, such as the Kuparuk Although thermally overmature where sampled, these River Formation and Kemik Sandstone. Interestingly, units contain sufficient organic material to have been fair Schenk and Bird (1992) speculated that the hydrocarbon source rocks. Tingmerkpuk sandstone represents a lowstand fan Crowder and others (1994) measured 131 m of derived from the Tunalik shelf region. Tingmerkpuk sandstone conformably overlying the Due to some uncertainty in the nature of the Kingak Shale on Tingmerkpuk Mountain. In this study, depositional system, T.N. Nilsen (Consultant), T.E. the Tingmerkpuk sandstone was interpreted as a Moore (U.S. Geological Survey), and M.D. Myers conformable succession that records episodic, highly (Alaska Division of Oil & Gas) remeasured the type fluidized turbidity currents that, over time, built a series section in 1996. Nilsen (1996) interpreted the of progradational outer fan lobes. This interpretation was Tingmerkpuk as the depositional record of repeated based on the presence of complete Bouma sequences, storm events in a mid- to outer-shelf setting. This conspicuous dewatering features, convolute lamination, interpretation is based on the presence of hummocky and graded bedding, sole markings, amalgamated beds, and swaley cross stratification, symmetrical wave ripple parasequence stacking patterns. The interpretation bedforms, wave ripple and wave-modified current ripple placed the Tingmerkpuk sandstone off the shelf in a cross lamination, trace fossils typical of shelf settings, basinal setting and fits the conventional paleogeographic and the general absence of Bouma sequences and other model for Beaufortian sandstones (that is, shallow water features characteristic of turbidites. Paleocurrent shelf and associated sandstones deposited along the indicators and hummocks overturned toward the east in

Upper Member

Middle Member

Lower Member Kingak Shale

Figure 1 (left). Generalized geologic map of the northwest DeLong Mountains, western Brooks Range, Alaska. Modified from Reifenstuhl (1997). Figure 2. View toward the northwest showing the type section of the Tingmerkpuk sandstone on the north flank of Tingmerkpuk Mountain. Geologist in the lower right corner of the photo (in circle) is standing near the base of the lower member. The prominent beds in the center of the photo belong to the middle member. The rubble-covered interval near the top of the slope is the upper member. Photo by Gil Mull. 48 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS convolute laminated beds suggest the dominant direc- Mountains, the LCu is commonly placed at the tion of storm movement was eastward, along the trend of Valanginian–Hauterivian boundary, but at some the outcrop belt (Nilsen, 1996). locations the surface is probably contained within the Nilsen’s interpretation does not fit well into the Hauterivian. On the south flank of the Barrow arch the existing paleogeographic model for Beaufortian LCu truncates rocks as young as the Kingak Shale; the sandstones. If the Tingmerkpuk was derived from a LCu cuts progressively downsection northward toward northern source, either associated with or located north the crest of the Barrow arch, where rocks as old as late of the Barrow arch, then the shelf upon which the Proterozoic have been truncated. Tingmerkpuk was deposited was at least 350 km wide. Numerous discontinuous shallow-marine, quartzose This is a conservative estimate that does not account for sandstone bodies of Neocomian age are associated with north-vergent thrusting that displaced the Tingmerkpuk the LCu. These are the sandstone bodies of local extent sandstone northward an unknown distance. Further, low- mentioned above. These sandstones have been angle south-dipping clinoforms in the Kingak Shale are recognized from the Aurora-1 well east of Barter Island clearly visible on seismic lines from the NPRA. These to the western Arctic Slope and Chukchi Sea. Some of reflectors indicate a ramp-like or low-angle slope that the better known include the Thomson sand, Put River deepened from the Barrow arch southward during Late sandstone, the Hauterivian to Barremian Kemik Jurassic to Early Cretaceous time. Sandstone (Mull, 1987; Reifenstuhl, 1995), and the Although biostratigraphic data indicate that the time Valanginian to Barremian Kuparuk River Formation equivalent of the LCu occurs within the Tingmerkpuk (Masterson and Paris, 1987). Lesser known sandstone sandstone, neither group found stratigraphic evidence for bodies include Valanginian sandstones in the Tunalik the unconformity. Crowder and others (1994) interpreted well and Neocomian sandstones on the Chukchi Sea east the contact between the Tingmerkpuk sandstone and the of Point Barrow. underlying Kingak Shale as the correlative conformity to A variety of contact relations with the LCu have been the LCu. recognized. Some sandstones are truncated by the LCu, others rest on the LCu, and still others are split by the REGIONAL STRATIGRAPHY LCu. Four members are recognized in the Kuparuk River Formation. The lower A and B members gradationally The Arctic Alaska terrane experienced a 100 m.y. overlie the Kingak Shale and are truncated by the LCu; interval of extension that lasted from Early Jurassic to the C and D members overlie the LCu. The Kemik Early Cretaceous (Hubbard and others, 1987). This Sandstone is present in discontinuous surface exposures interval is characterized by an Early Jurassic failed rift between the Sadlerochit and Echooka rivers in the Arctic episode, and an Early Cretaceous (Valanginian– National Wildlife Refuge (ANWR) and foothills of the Hauterivian) successful rift event which led to the northeastern Brooks Range, and extends into the opening of the Canada Basin (Grantz and May, 1983; subsurface west of the Echooka River. South of the Hubbard and others, 1987). Strata deposited during this Barrow arch, the LCu passes gradually into a correlative interval include the Jurassic to Lower Cretaceous conformity. South of the maximum southerly extent of (Valanginian) Kingak Shale, various Upper Jurassic to the LCu, Neocomian sandstones rest with apparent Hauterivian–Barremian sandstone bodies of local extent, conformity above the Kingak Shale. An example of this and the Hauterivian to Barremian Pebble Shale unit. conformable relation is seen along the Echooka River These rocks are referred to as the Beaufortian plate approximately 20 km southwest of the Kemik #1 well on sequence by Hubbard and others (1987). As in the the northeastern Arctic Slope near ANWR, where the underlying Ellesmerian sequence (lower Ellesmerian Kingak Shale is gradationally overlain by the Kemik sequence of Grantz and others, 1990), sandstones in the Sandstone. Northeast of this location, the Kemik rests Beaufortian plate sequence are quartzose and were unconformably above older rocks. derived from a northern source terrane north of the The contact between these Neocomian sandstones present day Arctic coast line. and the overlying Pebble Shale unit is sharp, but Uplift along the northern margin of the Arctic Alaska apparently conformable. Where the sandstones are terrane associated with Valanginian to Hauterivian rifting absent, the Pebble Shale unit rests directly on the LCu. created the Barrow arch and led to development of a In the northern DeLong Mountains, the Tingmerk- regional unconformity. This erosional surface, puk sandstone is exposed in thrust sheets along an 80- commonly referred to as the Lower Cretaceous km-long trend (fig. 1) (Reifenstuhl and others, 1997). unconformity (LCu), is considered the breakup Along this trend the Tingmerkpuk consists of a conform- unconformity by Grantz and May (1983). In the able succession of interbedded shale and sandstone subsurface north and northeast of the DeLong (Crowder and others, 1994). The Tingmerkpuk rests with MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)49 apparent conformity above the Kingak Shale and is STRATIGRAPHY AND SEDIMENTOLOGY O apparently conformably overlain by Brookian strata THE TINGMERKPUK SANDSTONE TYPE (fig. 3). Throughout much of its outcrop trend, two SECTION distinct facies belts of Tingmerkpuk sandstone have been recognized (Reifenstuhl and others, 1997): the southern The type section of the Tingmerkpuk sandstone is belt has been superimposed over the northern belt by located in the northern facies belt on the northern flank north-vergent thrusting. The northern facies belt is of Tingmerkpuk Mountain (figs. 1, 2). It contains a characterized by a higher sandstone:shale ratio and nearly continuous succession of sandstone and shale that coarser grained sandstones, whereas the southern facies lies with apparent conformity (in an exposed contact) belt has a lower sandstone:shale ratio and finer grained, upon the underlying Kingak Shale (fig. 4A). A detailed more thinly bedded sandstones. These observations measured section at a scale of 1:30 is available from suggest a relatively proximal depositional setting for the DGGS (LePain and others, 1999). northern belt and a more distal setting for the southern The upper 10–15 m of Tingmerkpuk consists largely belt. Unlike its correlatives in the subsurface and in the of rubble crop and the upper contact is a thrust fault. The northeastern Brooks Range, the Pebble Shale unit is thrust fault defines the base of a repeated section of absent and Brookian strata overlie the Tingmerkpuk uppermost Kingak and Tingmerkpuk sandstone (thrust sandstone with apparent conformity. repeat of the northern facies belt). The type section as we measured it consists of approximately130 m of interbedded sandstone and shale Torok Formation (fig. 4A). Crowder and others (1994) accurately describe the stratigraphic ALBIAN organization of the type section, which Mount Kelly Graywacke Tongue they divided into three members. Each of the Fortress Mountain Formation member is distinguished by the gross sandstone:shale ratio (increases upward), ? APTIAN sandstone bed thickness (average Lower Brookian Shale (> 300 m) thickness increases upward overall and within members), and the degree of amalgamation in sandstone bedsets (increases upward). Crowder and others (1994) viewed the contact between the Kingak Shale and the Tingmerkpuk sandstone as a sequence boundary.

CRETACEOUS Tingmerkpuk Sandstone (130 m) KINGAK SHALE

NEOCOMIAN The upper 27 m of the Kingak consists of dark gray clay shale to slightly silty clay shale. The Kingak is locally red weathering and includes thin beds of very fine-grained sandstone and small (few ? Kingak Shale (<500 m) centimeters in diameter) ovoid-shaped iron and/or manganese concretions. Overall the sandstone beds increase in number and thickness upsection from a

JURASSIC few millimeters near the base of the measured section (27 m below the Tingmerkpuk) to a few centimeters near Otuk Formation the contact with the Tingmerkpuk.

UPPER Superimposed on this overall increase in TRIASSIC the sandstone:shale ratio is a cyclic Figure 3. Generalized stratigraphic column for the northern DeLong variation in the abundance of sandstone Mountains, western Brooks Range, Alaska. No vertical scale is beds that defines meter-scale sandier- intended. 50 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS

A. E. 3 Additional 10 to 15 m of talus cover to top of slope

120

0

D. 7 100

Upper Member

Trip. Cht.

80 Tripolitic Chert ? 0

C. 2 60

Tingmerkpuk Sandstone

Middle Member

0 40

Lower Member B. 2 20

Kingak Shale 0

Meters Plane parallel lamination LEGEND Convolute lamination Wave ripple cross Dish structures Wavy parallel lamination lamination Groove cast Hummocky cross stratification Wave ripple bedforms Wavy lamination grading laterally Trace fossil to low-angle cross lamination Current ripple ()Planolites cross lamination Concretion

Figure 4. (A) Simplified measured stratigraphic section through the Tingmerkpuk sandstone at its type section on the northern flank of Tingmerkpuk Mountain. Due to the scale, individual parasequences are not shown. (B) Blowup showing typical event bed in the lower member. (C) Blowup showing two event beds near the base of the middle member; the vertical sequence of sedimentary structures shown is common, but is not the only vertical sequence observed. (D) Blowup showing two bedsets and one solitary bed. Each bedset is composed of at least two amalgamated beds; each bed represents a single depositional event. (E) Blowup showing a thick bedset in the upper member. Convolute lamination is the most common structure in the upper member; other structures observed are shown in figure 4A. MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)51 upward successions (fig. 4A). Sandstone beds typically Tool marks range from small, faintly visible (fig. 5A), have sharp basal contacts, are very subtly graded, and are discontinuous linear features to discontinuous groove horizontally laminated. In some beds horizontal laminae casts up to 2 cm wide and several centimeters long. grade laterally to single sets of ripple cross lamination. Discrete vertical burrows were not observed, but circular Some laminae drape ripple bedforms and pass laterally burrows up to 1.5 cm in diameter are present on the into foreset laminae. Cross laminae set bounding undersides of some sandstone beds (Planolites?), where surfaces are irregular and characterized by complex they are preserved in cross section parallel to bed sur- geometries. The basal portions of laterally adjacent sets faces. Rare pieces of sandstone float with casts of the of cross laminae are interleaved and one or more laminae pelecypod Buchia sublaevis were observed. Some casts commonly drape individual sets. The complex geometry included iridescent shell material. of set bounding surfaces, the geometry of the ripple cross The suite of sedimentary structures recognized in laminae sets, and the common presence of draping the sandstone beds include: (1) plane-parallel lamination laminae, resembles wave ripple cross lamination (less than 1 cm thick) (figs. 5B, 5C); (2) low amplitude illustrated by de Raaf and others (1977). (less than 2 to 3 cm) wavy, parallel lamination (fig. 5B); (3) small-scale hummocky cross stratification (HCS) LOWER MEMBER with wavelengths from 20 cm to approximately 50 cm and amplitudes from a few centimeters to approximately DESCRIPTION 8 cm (figs. 5C, 5D, 5E); (4) wave or wave-modified The contact between the Kingak Shale and the current ripple cross-lamination; (5) wave ripple bed- overlying Tingmerkpuk sandstone is sharp and placed at forms (fig. 5F); (6) convolute lamination; and (7) parting the base of the first prominent sandstone bench on the lineation. Parting lineation was observed in plane- northwest side of Tingmerkpuk Mountain (fig. 4A). A parallel laminated beds. Beds with HCS consist of 10-cm-thick sandstone bed at the base of the bench laminae dipping at low angles (usually less than 10o) consists of medium to dark gray, very fine-grained relative to truncation surfaces and laminae draping low- argillaceous sandstone that rests above a sharp erosive relief hummocks. Symmetric versus asymmetric forms contact with the underlying Kingak Shale. This 10-cm- of HCS were not distinguished. The undisturbed thick bed forms the base of a 4.2-m-thick coarsening laminae geometry is difficult to determine in convolute upward bedset that comprises the sandstone bench (fig. laminated intervals. However, low-angle laminae 4A). The lower member extends from the base of the truncations are common, suggesting hummocky cross sandstone bench at 27 m to the top of the first bedset of stratification. Plane-parallel lamination and wavy amalgamated sandstone beds (above the sandstone lamination are typically the most common structures in bench) at 49.5 m. the thinner beds; HCS is present, but is not as common. Above the sandstone bench at 27 m, the lower Lateral gradations between plane-parallel lamination, member consists of interbedded slightly silty dark gray wavy lamination, and hummocky cross stratification are to black shale and very fine to fine grained quartzose common in many beds. The degree of deformation in sandstone. The lower member is shale dominated and convolute laminated intervals typically decreases has a sandstone:shale ratio less than 0.5. The shales abruptly upsection and gradationally parallel to bedding. include thin (less than 2 cm) bentonite beds and ovoid- Folds within convolute laminated intervals are shaped iron and manganese (?) concretions. Most commonly overturned toward the east. sandstone beds are very fine grained, medium gray to Several meter-scale sandier-upward parasequences red-brown weathering, and are medium to light gray on have been recognized in the lower member. Each fresh surfaces. Sandstone beds range from less than 1 cm parasequence is characterized by a gradual increase in to approximately 0.5 m thick. The thinner beds are the number and thickness of sandstone beds upsection. characterized by sharp upper and lower contacts, The stratigraphic organization of the lower member is whereas the thicker beds have sharp basal and similar to the organization of the uppermost Kingak gradational upper contacts with enclosing shale. The Shale, except that in the latter the sandstone:shale ratio amalgamated beds at the top of the member have and average thickness of the sandstone beds are much undulatory basal contacts. less. If not for the bedset between 27 and 31.2 m, which Horizontal trace fossils similar to Planolites and is markedly different from any of the sandstones linear tool marks are abundant on the undersides of many observed in the Kingak, the lower member would simply sandstone beds (fig. 5A). Horizontal trace fossils range grade downsection into the Kingak Shale. The low from a few millimeters wide to robust forms up to 1 cm sandstone:shale ratio and absence of sand-on-sand wide; horizontal trace fossils are preserved in semi-relief, contacts (amalgamation surfaces) suggest an aggrada- usually have a meandering plan form, and rarely branch. tional stacking pattern for the lower member. 52 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS

A B

C D

E F

Figure 5. Sedimentary structures in sandstones from the lower member. (A) Trace fossils preserved on the underside of a sand bed in the lower member. Some traces resemble Planolites. Low-relief tool marks are also visible. (B) Plane-parallel lamination and low-relief wavy lamination in a sandstone bed from the lower member. The underside of this bed contains Planolites traces. A low-relief hummock grades upward to plane-parallel lamination immediately above the geologist’s finger. (C) Plane-parallel lamination, low-relief wavy lamination, and possible hummocky cross-stratification (HCS) in a sandstone bed from the lower member. Plane-parallel laminae are visible in the lower left corner of the bed, which is overlain by a low-relief convex upward hummock; laminae in the hummock downlap on plane-parallel laminae. The hummock grades upward to low-relief wavy laminae which, in turn, grade upward to plane-parallel laminae. (D) Small-scale (wavelengths range from 15 to 25 cm) HCS in a sandstone bed from the lower member. HCS grades upward to plane-parallel laminated sandstone. (E) Small-scale HCS in sandstone bed from the lower member. (F) Wave ripple bedform preserved on the surface of a sandstone bed in the lower member. MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)53

MIDDLE MEMBER sandstone beds. Convolute lamination is more abundant in the middle member than in the lower member. Parting DESCRIPTION lineation was only observed in plane-parallel laminated The middle member extends from 49.5 m to 77.2 m, beds. Small shale ripups were noted in a sandstone bed consists of interbedded shale and sandstone, and is at 72.1 m. Plane-parallel lamination, wavy lamination, distinguished from the lower member by a higher and convolute lamination are the most common sandstone:shale ratio (greater than 3), greater average sedimentary structures (figs. 6A, 6B). A number of sandstone bed thickness, and the presence of different vertical sequences have been recognized in amalgamated sandstone beds. The latter two criteria are sandstone beds of the middle member; however, the most important for separating the lower and upper hummocky cross stratification and wavy parallel members. The stratigraphic organization recognized in lamination is most commonly present near the base of the middle member can be viewed as the continuation of beds and usually grades upsection to convolute a vertical trend (that is, increasing sandstone upsection) lamination or wave ripple cross lamination. that started in the uppermost Kingak Shale. Owing to the higher sandstone:shale ratio and greater The shales are slightly silty, with only minor “clean” average sandstone bed thickness, individual clay shale intervals, and include common iron and parasequences are less obvious in the middle member manganese (?) concretions, and minor thin (less than than in the lower member. The relatively uniform 1 cm) beds of pale yellow weathering bentonite. Most interbedding of sandstone and shale defines an sandstones are very fine- to fine-grained, weather light aggradational stacking pattern for the middle member; gray to light tan-brown, and are light colored on fresh the appearance of amalgamated beds near the top of the surfaces. Sandstone beds range from less than 1 cm to member indicates a decrease in accommodation space greater than 1 m thick. The thinner sandstone beds have relative to the lower member. sharp contacts with the enclosing shales. Thicker beds that are enclosed in shale have sharp, erosive lower UPPER MEMBER contacts and gradational upper contacts. Bedsets of amalgamated sandstone beds are common and occur DESCRIPTION throughout the member. The basal surface of most The upper member extends from 77.2 m to the top of thinner sandstone beds is flat, whereas most thicker beds the section at 140 m. Approximately 50 percent of the have undulatory bases. Trace fossils and sole marks thickness of the upper member is sandstone talus and similar to those described in the lower member are minor shale fragments. The exposed 50 percent consists common. of very fine- to fine-grained quartzose sandstone, minor Trace quantities of glauconite are visible on fresh medium grained sandstone, and minor black to dark gray, surfaces of many sandstone beds. Notably, granule-sized slightly silty shale. The upper member is distinguished clasts of tripolitic (leached) chert are present at the base from the middle member by the increase in bed of the sandstone bed at 68.5 m. The granules are floating thickness, the “cleaner” appearance of the sandstones, within a “matrix” of sand-sized grains. This is the lowest and the apparent reduced thickness of interbedded shale. stratigraphic occurrence of visible tripolitic chert Sandstones are light tan to tan-brown weathering and recognized thus far in the Tingmerkpuk type section. The light tan on fresh surfaces (fig. 6E). Visible basal bed reader is referred to Reifenstuhl and others (1997) for a contacts are sharp and flat over the scale of the outcrop; detailed discussion of the petrography of the both gradational and sharp upper bed contacts have been Tingmerkpuk sandstone. observed. Bedding in the sandstones is faint to invisible The suite of sedimentary structures recognized in and, where visible, ranges from 0.5 m to more than 1 m sandstone beds include: (1) plane-parallel lamination thick. Where internal structures are visible, plane- ranging from a few millimeters to 2 cm thick (figs. 6A, parallel lamination is most common (fig. 6F). No trace 6B), (2) low amplitude (from less than 2 to 3 cm) wavy fossils or sole marks were observed on the undersides of parallel lamination, (3) small-scale HCS with sandstone beds. wavelengths from a 20 cm to approximately 50 cm, and Glauconite is visible in trace quantities on fresh amplitudes from a few centimeters to approximately surfaces in many sandstone beds. Sand- to granule-size 8 cm, (4) wave ripple cross lamination, (5) wave- tripolitic chert grains were observed in the lowermost modified current ripple cross lamination (fig. 6C), bed (77.2 m) of the member (figs. 4A, 6E). This bed (6) wave ripple bedforms with wavelengths from 10 to represents the uppermost of two occurrences of 25 cm and amplitudes to 4 cm, (7) convolute lamination megascopic tripolitic chert in the Tingmerkpuk type (figs. 6A, 6B), and (8) parting lineation (fig. 6C). Wave section. The reader is referred to Reifenstuhl and others ripple bedforms (symmetric to slightly asymmetric (1997) for a detailed discussion of the petrography of the profiles) are present in float and on the surface of some Tingmerkpuk sandstone. 54 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS

Figure 6A. Convolute laminated bed in Figure 6B. Convolute laminated interval the middle member. Convolute abruptly overlain by a plane-parallel laminated interval is abruptly over- laminated sandstone in the middle lain by plane-parallel laminated member. The convolute laminae grade sandstones; the convolute laminated upward to incipient convolute laminae interval is underlain abruptly by which, in turn, are abruptly overlain sandstone with small-scale HCS. by plane-parallel laminae. This bed is Determining whether this succession interpreted as the product of a single is a bedset of amalgamated beds or a depositional event. single bed deposited during one event is difficult; the lack of evidence for erosion along the surfaces bounding the convolute laminated interval suggests that the succession records a single depositional event.

Figure 6C. Wave ripple or wave-modified (combined flow) cross lamination in a sandstone bed from the middle member. The foreset laminae are bounded by plane- parallel laminated sandstones. Foreset laminae overlie an erosional surface characterized by complex geometry; foreset laminae are grouped in bundles that are bounded by complexly shaped surfaces. The ripple cross lamination resembles wave-generated cross lamination shown by de Raaf and others (1977).

Figure 6. Sedimentary structures in sandstones from the middle and upper members. MEASURED SECTION AND INTERPRETATION OF THE TINGMERKPUK SANDSTONE (NEOCOMIAN) 55

Figure 6D. Parting lineation on the underside of a Figure 6E. Light colored “clean” sandstone at the base sandstone bed in the middle member. Parting of the upper member. Note the rubbly character of the lineation indicates high energy conditions, exposure. Geologist is examining the tripolitic chert- equivalent to the upper flow-regime in bearing bed at the base of the member. unidirectional flows.

Figure 6F. Light colored “clean” sandstones with wavy Figure 6G. Convolute laminated sandstone beds in the lamination in the upper member. Wavy laminae upper member. grade upward to plane-parallel lamination.

The suite of sedimentary structures recognized in STRATIGRAPHY OF THE SOUTHERN FACIES sandstones include plane-parallel lamination (0.5 cm to BELT OF THE TINGMERKPUK SANDSTONE 2 cm thick), wavy parallel lamination, small-scale HCS (similar size range as recognized lower in section), The southern facies belt can be traced for a number convolute lamination, and dish structures. Apparently of kilometers along strike (fig. 1). Brief reconnaissance structureless sandstones are also present in the upper of this belt was conducted at a location 3 km south- member. Convolute lamination is the most common southwest of Tingmerkpuk Mountain. Owing to sedimentary structure recognized in the upper member, relatively poor exposures, no sections of the and the structure is ubiquitous in many beds (fig. 6G). No Tingmerkpuk in this belt have been measured, but its recurring vertical sequence of sedimentary structures has thickness and stratigraphic organization appears to be been recognized. comparable to the Tingmerkpuk in the northern belt. Abundant amalgamated beds, relatively uniform Petrographic study of the Tingmerkpuk in the southern grain size in sandstones, and the lack of exposed shales facies belt shows no significant difference in framework preclude identification of individual parasequences. The grain composition relative to the northern facies belt ubiquitous presence of amalgamated sandstone beds (Reifenstuhl and others, 1997). Megascopic tripolitic indicates a significant reduction in accommodation chert was not observed at exposures of the Tingmerk- relative to the lower and middle members. The contact puk in the southern facies belt. between the upper and middle members is interpreted as Exposures of the southern facies belt of the a sequence boundary. Tingmerkpuk sandstone are present on the slope 56 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS immediately southwest of a northwest-flowing tributary upper-flow regime conditions (Simons and others, to Eagle Creek. Bedding is nearly vertical to slightly 1965; Harms and others, 1982), strong oscillatory flows overturned toward the south. Tundra covered shale (Arnott and Southard, 1990), and from oscillatory- intervals are interrupted by resistant sandstone bedsets dominant combined flows (Arnott and Southard, 1990). ranging from less than 1 m to 2 m thick. The dominant Lateral gradations observed within individual beds sedimentary structures are plane-parallel lamination, range from plane-parallel lamination to wavy lamination low-relief wavy lamination, and convolute lamination. and HCS; these structures and the presence of wave- Most wavy laminae are not perfectly parallel, but do not modified current-ripple cross lamination near the tops of intersect. The most common vertical sequence observed some of these beds suggests deposition from oscillatory- in individual bedsets is plane-parallel lamination and dominant combined flows. Laboratory flume studies low-relief wavy lamination near the base that grades show that plane-parallel laminae result from the super- upsection to convolute lamination. Members have not position of weak unidirectional currents on strong been identified in the Tingmerkpuk at this location, but oscillatory flows (Arnott and Southard, 1990). The sandier-upward successions are apparent. presence of parting lineation in plane-parallel laminated sandstone beds attests to deposition from high-energy INTERPRETATION O THE TINGMERKPUK flows and is commonly associated with storm-generated SANDSTONE AT TINGMERKPUK MOUNTAIN sandstone beds (Leckie and Krystinik, 1989; Cheel, 1990). LOWER MEMBER INTERPRETATION Subtle size grading recognized in many of the plane- Thin sharp-based sandstone beds in the lower parallel laminated beds suggests the presence of sand- member record episodic, high-energy events that trans- sized material in suspension above the sea floor. Graded ported coarse-grained sediment into a mud-dominated bedding has been observed near the base of many storm- depositional setting. The low sandstone:shale ratio generated beds (for example, Aigner, 1985; Dott and indicates that low energy conditions prevailed and high Bourgeois, 1982; Kreisa, 1981; and Walker and others, energy events were relatively infrequent. During low 1983) and, in distal settings, may comprise the entire energy conditions, clay- and fine silt-sized particles thickness of storm beds. Some graded plane-parallel settled out of suspension to build the relatively thick laminated beds recognized in the lower member interbedded mudstone successions. Hummocky cross resemble graded rhythmites described by Dott and stratification, wave ripple bedforms and cross lamina- Bourgeois (1982) and Myrow (1992). tion, and wave-modified current ripple cross lamination The origin of the wavy lamination is unclear. Arnott suggest deposition from oscillatory, or oscillatory- (1987, cited in Arnott and Southard, 1990), de Raaf and dominant combined flows that operated between fair- others (1977), and Kreisa (1981) describe gently weather and the storm wave base associated with undulating parallel-laminated sandstones that resemble powerful storms. The presence of thick interbedded the low-amplitude wavy lamination in the lower mudstone successions indicates that the sediment-water member. In flume studies of oscillatory and combined interface was only occasionally affected by wave activity. flows, Arnott and Southard (1990) noted that purely Some of the storm-generated flows were able to oscillatory bedforms were quickly planed off forming a scour into the muddy substrate as indicated by the gently undulating bed after a relatively weak Planolites burrows preserved in semi-relief on the unidirectional current was superposed on a strong undersides of many sandstone beds. In tiered trace fossil oscillatory flow. Based on observations from their flume assemblages, Planolites usually occupies a shallow tier studies, Arnott and Southard (1990) suggested a below, but close to, the sea floor (Bromley, 1996). combined flow origin for the gently undulating parallel Planolites is associated with well-oxygenated to slightly lamination observed in the stratigraphic record. The dyoxic sediment pore waters (Bottjer and others, 1986). wavy parallel lamination in the lower member may have Rare circular burrows, preserved on the undersides of a similar origin. Alternatively, the wavy parallel some sandstone beds, may represent escape structures lamination in the Tingmerkpuk may record deposition produced shortly after the beds were emplaced. Linear from clouds of suspended sediment under strong groove marks preserved on the undersides of many oscillatory flows resulting in vertical accretion of the sandstone beds indicate that flows were strong enough to wavy bedform with little or no horizontal translation. In transport objects up to 2 cm wide. either case, if the bedform responsible for the wavy Plane-parallel lamination is the most common parallel lamination in the Tingmerkpuk consisted of sedimentary structure in the lower member. Plane- three-dimensional hummocks, it may be genetically parallel lamination may develop from several different related to HCS. Lateral gradation from wavy lamination types of flows, including unidirectional currents under to plane-parallel lamination, and from wavy lamination MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)57 to hummocky cross stratification (middle and upper as a form of antidune stratification that developed under members) strongly suggests a genetic connection standing waves formed at the boundary of a denser, between these structures. Lateral gradations between thinner underflow and an overlying, less dense layer in structures indicate spatial variations in the flow strength a basinal setting. Prave and Duke (1990) suggest that and the concentration of suspended sediment near the small-scale hummocky cross stratification is multi- sea bottom. genetic and not indicative of a specific depositional Convolute laminated beds may have formed shortly environment or hydrodynamic condition. after deposition and possibly during the event The geometry of the HCS observed in the lower responsible for deposition of the bed. The relatively member is similar in every aspect to examples of larger- consistent direction of eastward asymmetry observed in scale HCS reported in the literature (for example, Dott the convolute laminated intervals suggests that either the and Bourgeois, 1982; Harms, 1979; and Walker and storm-generated currents exerted enough frictional drag others, 1983), which suggests a similar origin— on the bed surface to cause overturning in the direction deposition and reworking from powerful storm- of the dominant bottom current (Nilsen, 1996), or that the generated oscillatory flows. The association of sand beds were deposited on a surface that sloped toward hummocky cross stratification and wave ripple cross the east. We prefer the former possibility. lamination in sandstones from the lower member The gradational tops of many of the thicker sandstone strongly suggests a storm origin and indicates deposition beds indicate gradually waning flow strength and above storm wave base. This association makes a suspension fallout. Gradational upper contacts may also standing wave antidune origin unlikely. have been accentuated by the activities of burrowing We speculate that the small size of the hummocky organisms. As mentioned in the description for this cross stratification in the Tingmerkpuk is the result of member, discrete burrows were not observed within deposition at water depths near storm wave base, in a sandstone beds. This may reflect complete bioturbation subsiding outer-shelf setting. If the bedform responsible of the uppermost portion of sand beds in the months to for HCS is correctly classified as an orbital ripple, then years following deposition. the bedform wavelength is related to the near bottom All of the HCS observed in the Tingmerkpuk orbital diameter of the associated waves. This implies sandstone ranges from a few decimeters to 5 dm in that at the exact depth that storm waves first begin to wavelength, and as such would be classified as “small- interact with the sea floor, the near-bottom orbital scale.” Small-scale HCS recognized in the Tingmerkpuk diameter is zero. At depths close to but less than storm sandstone warrants further discussion. Campbell (1966) wave base, the near-bottom orbital diameter is small and described the wavelength of HCS (his truncated wave resulting bedforms have small wavelengths. A ripple lamination) as ranging from a few decimeters to paleohydraulic analysis of HCS by Duke and Leckie several meters. Most recent literature on shelf storm (1986) supports this reasoning. From their analysis deposits list a lower wavelength limit of approximately Duke and Leckie (1986) concluded that hummocky 0.5 m for HCS (Harms, 1979; Mount, 1982; Myrow, cross stratification forms under oscillatory-dominant 1992; and Walker, 1984). However, Cheel and Leckie flow from three-dimensional orbital ripples with (1993) point out that there is no physical basis for the wavelengths approximately equal to the near-bottom 0.5 m lower size limit on the wavelength of HCS. Dott orbital diameter. and Bourgeois (1982) and Brenchley and others (1986) In marine shelf settings we do not believe that HCS describe examples of small-scale HCS interpreted to is genetically related to orbital ripples. Clifton (1976) record deposition from strong oscillatory flows originally introduced the term orbital ripple and stated associated with storms in marine shelf settings. Higgs that they are associated with short period waves typically (1991 and 1998) describes small-scale HCS from the encountered in relatively protected shallow-water Carboniferous Bude Formation in southwest England, settings. For waves to entrain fine sand at water depths which he interprets as a storm-influenced lacustrine shelf typically encountered in middle-to outer-shelf settings, deposit. Although the lacustrine setting of the Bude long period waves similar to those encountered on Formation is fundamentally different than the shelves facing deep oceans are required. Bedforms Tingmerkpuk’s marine setting, like the Tingmerkpuk, associated with long period waves, referred to as controversy surrounds its interpretation as many workers anorbital ripples by Clifton (1976), have wavelengths consider it to be a lacustrine turbidite succession (Burne, that are unrelated to the near-bottom orbital diameter of 1995; Burne, 1998; Melvin, 1986). the waves. Prave and Duke (1990) described decimeter-scale Regardless of the correct classification of the HCS HCS associated with turbidites in Upper Cretaceous bedform (orbital or anorbital), it seems reasonable to strata in the Basque Pyrenees. They interpreted the HCS assume that for a given size surface wave, the mean 58 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS wavelength of oscillatory bedforms will decrease as frequent. The change in stratigraphic organization from water depth increases. If this assumption is correct, the lower to the middle member suggests a decrease in small-scale HCS should be generated near mean storm accommodation, and a corresponding increase in the wave base and the scale of HCS should gradually progradation rate. increase with a gradual decrease in water depth, at least to within the wavelength range commonly observed in UPPER MEMBER INTERPRETATION ancient shelf successions (0.5m to a few meters The suite of sedimentary structures recognized in the wavelength). In many ancient middle-to outer-shelf upper member is similar to that recognized in the lower settings, mean storm wave base was probably situated two members, and is similarly interpreted. Thick above the sea floor (as it is in most Modern settings). amalgamated intervals in the upper member and the Consequently, in these settings only waves associated apparent lack of interbedded shales are suggestive of with the most intense storms were able to entrain fine deposition under conditions of significantly reduced sand-sized sediment. Wave base associated with the most accommodation. The mature quartzose sandstones with intense storms would be slightly deeper than mean storm convolute lamination and dish structures attest to rapid wave base. The preserved recurrence interval for these deposition of individual beds. There are three possible intense storms may have been greater than 1,000 years explanations for the apparent lack of mudstone: (see Walker, 1985, p. 294). By analogy, the typical (1) background energy levels were too high for fine- tempestite bed in the Tingmerkpuk may represent the grained material to settle out of suspension; 1,000-year or 2,000-year storm event, and may record (2) deposition occurred well above storm wave base, and deposition slightly below mean storm wave base. episodic high energy storm flows were frequent and Based on the analysis presented above and removed most mudstone; or (3) mudstones are present, paleobathymetry (outer neritic to bathyal) estimates from but are covered by sandstone talus. Abundant foraminifera recovered from the base of the Tingmerk- amalgamation surfaces, the ubiquitous presence of puk and the uppermost beds of the Kingak Shale (Mickey “clean” sandstones, and the presence of at least one and others, 1995), we infer an outer-shelf setting for the mudstone interval (at 123.7 m) suggests a combination lower member. Thick intervening silty shales and the of the second and third explanations. progressive thickening of sandstone beds at the top of successively higher members defines an aggradational to INTERPRETATION O THE TINGMERKPUK slightly progradational stacking pattern. SANDSTONE IN THE SOUTHERN ACIES BELT Similar depositional processes are inferred for the upper 27 m of the underlying Kingak Shale. The deposi- Individual sandstone beds represent episodic events. tional setting was likely deeper and less storm-disturbed Unlike the section at Tingmerkpuk Mountain, wave- than that recorded in the lower member of the formed and wave-modified structures were not observed Tingmerkpuk sandstone. Another possibility is that the at the site visited in the southern facies belt. This may uppermost Kingak reflects deposition lateral to the lower reflect the fact that the section was not examined in detail, member (lateral facies change) in a middle-to outer-shelf or it may reflect a more distal depositional setting that setting, and is not a proximal to distal relation. was below storm wave base. The latter case seems more likely and by analogy with the northern facies belt, the MIDDLE MEMBER INTERPRETATION event beds at this location likely record deposition from The suite of sedimentary structures recognized in the storm-generated flows below storm wave base. Two middle member is similar to that observed in the lower types of storm-generated flows can potentially transport member and is similarly interpreted as the record of sand on shelves in water depths below storm wave base: storm-influenced sedimentation. The scale of hummocky (1) geostrophic currents (Swift, 1985) and (2) turbidity cross stratification in the middle member appears similar currents (Walker, 1984; Myrow and Southard, 1996). At to that observed in the lower member, which is inferred present it is not possible to distinguish between these two to have formed near, but above, storm wave base. If the flow types in the southern facies belt. organization of the middle member reflects a shoaling trend, HCS with wavelengths more typically observed in DISCUSSION shelf settings (0.5 m and larger) would be expected. However, convolute lamination is more abundant in the The inferred outer-shelf depositional setting for the middle member than in the lower member, which could Tingmerkpuk sandstone is based on the presence of obscure an increase in the wavelength of HCS. HCS, wave ripple cross lamination, wave-modified The appearance of amalgamated beds suggests that current ripple cross lamination, and wave ripple storm events impacting this part of the shelf were more bedforms. Although these features are not the most MEASURED SECTION AND INTERPRETATION O THE TINGMERKPUK SANDSTONE (NEOCOMIAN)59 common sedimentary structures recognized in the type bearing coarse-grained sandstone at the base of the upper section, their significance far outweighs their abundance. member to mark an intraformational unconformity These structures are wave-formed or, at least wave- (sequence boundary). The distance from the Barrow arch modified, and indicate deposition above storm wave (rift shoulder) suggests that this unconformity is base. The depth of storm wave base is dependent on the unrelated to the LCu (C. Paris, oral commun.). overall wave climate in a particular basin. Wave climate is influenced by many factors, including the shape, size, SPECULATION ON NEOCOMIAN and depth of the basin, as well as the basin’s paleolatitude PALEOGEOGRAPHY and the size and location of surrounding land masses. Storm waves capable of moving fine-grained sand have The present location of Tingmerkpuk sandstone been reported in water depths greater than 200 m on the outcrops in the western DeLong Mountains is 350 km Oregon coast (Komar and others, 1972). Bathymetry south of the Barrow arch. With only one exception, the estimates from foraminifera recovered from the inferred depositional setting for these sand bodies ranges uppermost beds of the Kingak and basal Tingmerkpuk from marginal- to shallow-marine environments on the (Mickey and others, 1995) suggest deposition in water at south flank of the Barrow arch. The single exception is least 100 m deep. This is consistent with deposition in an a thin 0.5-m- to 2-m-thick bed of quartzose sandstone outer-shelf setting. As mentioned previously, mean storm referred to as the Kemik Sandstone member of the wave base was likely above the sea floor along the Kongakut Formation that is extensively exposed in the Tingmerkpuk trend and only the most intense storms western Bathtub Ridge area in the eastern Brooks Range (1,000+-year events) were recorded in the succession. (Camber and Mull, 1987; Mull, unpublished field data, The overall organization of the Tingmerkpuk sand- 1985). Based on composition and stratigraphic position, stone is regressive, suggesting basinward progradation this Kemik bed is considered a deep-water equivalent of of the depositional system. If our reasoning regarding the Kemik Sandstone, which is exposed from the the wavelength of hummocky cross stratification Echooka River to Ignek Valley in the northeastern preserved in the Tingmerkpuk is correct, and if the Brooks Range. Tingmerkpuk records gradually shoaling conditions, The paleogeographic implications of an outer-shelf then a progressive increase upsection in the average interpretation for the Tingmerkpuk sandstone are wavelength of HCS should be recorded. We hypothesize difficult to reconcile if the source terrane was the Barrow that the wavelength increases upsection to within the arch or some landmass north of the arch. For the Barrow range typically observed in shallow-marine storm arch to have been the source, sand would have had to deposits (0.5 m to 2 m). This hypothesis should be tested travel at least 350 km across a broad shallow-marine in future work on the Tingmerkpuk. shelf and intervening slope and basinal area to the The significance of the tripolitic chert in the depositional site. This estimate does not account for the sandstone bed at 68.5 m and at the base of the upper shortening recorded by thrust faults mapped along the member (bed at 77.2 m) is unknown. Tripolitic chert is Tingmerkpuk outcrop belt. In fact, the location of the abundant in the Ivishak Formation, immediately below Tingmerkpuk so far from the inferred source along the the LCu along the Barrow arch; is present but less Barrow arch is probably one of the factors that initially abundant in the Ivishak Formation south of the Barrow led Crowder and others (1994) to suggest deposition in arch; and is also present in the Kemik Sandstone a basinal setting. We view the Barrow arch as an unlikely overlying the LCu (Mull, 1987). The appearance of source for the Tingmerkpuk sandstone. megascopic tripolitic chert in the Tingmerkpuk may be A northern source located south of the Barrow arch related in some way to development of the LCu and would significantly reduce the distance from the source erosion of chert-bearing older lithologic units (Ivishak terrane to the depositional site. Schenk and Bird (1992) Formation and Lisburne Group). Interestingly, the two show a paleogeographic reconstruction of the North occurrences of tripolitic chert recognized in the Slope and Barrow arch at the maximum development of Tingmerkpuk are located at, and just below, the base of the LCu during Valanginian–Hauterivian time. In their the upper member. The base of the upper member reconstruction, a broad lowland coastal plain and narrow corresponds to a significant change in stratigraphic shelf are shown on the south flank of the Barrow arch. If organization characterized by “clean” sandstone and this lowland area served as a source for terrigenous thick, amalgamated bedsets. These bedsets are either clastic sediment, then the distance between the source convolute laminated or appear structureless. Common area and the depositional site in the DeLong Mountains dish and pillar structures in these beds suggest extensive would be significantly less (possibly less than 250 km). dewatering. We interpret the abrupt appearance of In fact, Schenk and Bird (1992) suggested a source for megascopic tripolitic chert in granule- and pebble- the Tingmerkpuk sandstone in the vicinity of the Tunalik 60 SHORT NOTES ON ALASKA GEOLOGY 1999 LEPAIN AND OTHERS well. The LCu has been recognized in the Tunalik well, ACKNOWLEDGMENTS where the surface truncates Neocomian sandstones. If the Tingmerkpuk was derived from erosion of older Neo- We thank Kirk Sherwood and Mark Myers for their comian sandstones in the Tunalik area, a mechanism is reviews of this manuscript. necessary to bypass the intervening slope and basinal areas. We view the Tunalik area as an unlikely source for REERENCES the Tingmerkpuk sandstone. 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J.C., 1986, Proximal and distal hummocky cross- Coeval strata in the Okpikruak Formation south and stratified facies on a wide Ordovician shelf in Iberia, southeast of the Tingmerkpuk thrust sheets include in Knight, R.J., and McLean, J.R., eds., Shelf sands only sandstones and conglomerates that are highly and sandstones: Canadian Society of Petroleum lithic. On petrographic grounds, a southern source Geologists, Memoir 11, p. 241–255. appears unlikely. Bromley, R.G., 1996, Trace fossils—biology, taph- A fourth possibility is that the Tingmerkpuk onomy and applications (second edition): London, sandstone was derived from an unrecognized Chapman & Hall, 361 p. paleotopographic high in what is now the southern Burne, R.V., 1995, The return of “The Fan That Never Colville basin. If such a high exists, it is probably now Was”—Westphalian turbidite systems in the Variscan buried beneath the thrust sheets of the western Brooks Culm basin—Bude Formation (southwest England), Range. 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Division of Geological & Geophysical Surveys Walker, R.G., Duke, W.L., Leckie, D.A., Dott, R.H., Jr., Professional Report 117, p. 53–67. and Bourgeois, Joanne, 1983, Hummocky Reifenstuhl, R.R., Mull, C.G., and Wilson, M.D., 1997, stratification—significance of its variable bedding Petrography of the Tingmerkpuk sandstone sequences—discussion and reply: Geological Society (Neocomian), northwestern Brooks Range, Alaska— of America Bulletin, v. 94, no. 10, p. 1245–1251. STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE, NUTZOTIN AND MENTASTA MOUNTAINS, ALASKA Jeffrey D. Manuszak1 and Kenneth D. Ridgway1 ABSTRACT

The Nutzotin Mountains sequence is a 2,000-m-thick sequence of predominantly siliciclastic deposits that filled the Upper Jurassic to Lower Cretaceous Nutzotin basin. The Nutzotin basin is part of a series of deformed sedimentary basins located between rocks representing the Mesozoic continental margin of western North America and rocks representing an assemblage of allochthonous terranes that were accreted to the continental margin. Twenty-one measured stratigraphic sections define three facies associations that we use to develop a stratigraphic framework for the Nutzotin basin. The stratigraphically lowest facies association (FA1) consists of a basal conglomerate with outsized limestone clasts capped by a thick sequence of black shale with minor red, calcareous mudstone and conglomerate. Outsized limestone clasts of FA1 are on the order of tens of meters in diameter. Paleoflow indicators from FA1 are to the northwest. The next facies association (FA2) forms the bulk of the sequence, and consists of interbedded siltstone and shale with subordinate amounts of channelized pebble conglomerate and sandstone. Greenstone, limestone, granite, and chert clasts are common in conglomerates of FA2. Paleoflow indicators from FA2 are to the east and northeast. The stratigraphically highest facies association (FA3) consists of fossiliferous mudstones that grade upward into an overlying volcanic sequence.

Stratigraphic correlation of the three facies associations throughout the Nutzotin basin shows the following general trends. FA1 is best developed along the southern margin of the Nutzotin basin. FA2 is the most widespread facies association and is especially well developed along the axis of the basin. FA3 is restricted to the southeastern margin of the basin and appears to be closely related to the overlying Chisana volcanic strata. We interpret the stratigraphic architecture defined for the Nutzotin basin to represent a basin configuration in which local coarse-grained submarine fan systems (FA1) prograded northward transverse to the axis of the basin. A large axial submarine fan system (FA2) transported mainly sand and mud eastward and filled a large part of the basin. FA3 represents the final stages of sedimentary deposition in the Nutzotin basin before the onset of widespread Chisana volcanism. This facies association was probably deposited on a shallow muddy shelf, rich in marine invertebrates, that flanked the growing Chisana volcanic arc in the southeastern corner of the Nutzotin basin.

INTRODUCTION

The accretion of allochthonous terranes to the basin, one of the northernmost basins in the belt (fig. 1), Mesozoic continental margin of western North America however, has undergone only minor metamorphism was an important process in the growth of the North (Berg and others, 1972; Richter, 1976; Kozinski, 1985). American continent (Coney and others, 1980; Burchfiel The deposits of the Nutzotin basin consist of at least 2 km and others, 1992; Plafker and Berg, 1994; Nokleberg and of rhythmically bedded mudstone, siltstone, sandstone, others, 1994). Sedimentary basins that form coeval with conglomerate, and impure limestone (Berg and others, accretionary events commonly contain a detailed record 1972) deposited in a submarine fan deposystem (Kozinski, of accretionary processes (Lundberg and Dorsey, 1988; 1985; Manuszak and others, 1998). McClelland and others, 1992). For example, the dis- Our report is the first detailed stratigraphic analysis continuous belt of Jurassic–Cretaceous sedimentary of the entire Nutzotin Mountains sequence. We present basins exposed from Washington to Alaska (fig. 1) stratigraphic data from 21 measured sections throughout records the collision, uplift, and denudation of alloch- the Nutzotin and Mentasta mountains (fig. 2), which we thonous terranes with the North American margin (Rubin use to develop a more complete basin-wide stratigraphic and Saleeby, 1991; McClelland and others, 1992; Cohen architecture. Development of a stratigraphic framework and others, 1995; Kapp and Gehrels, 1998). for the relatively undeformed Nutzotin Mountains Unfortunately, most of the basinal strata within this belt sequence will provide a better understanding of the are highly metamorphosed and structurally complex, original basin configuration. Additionally it will serve thus the original depositional setting and structural as a template for analysis of correlative sedimentary configuration of the basins are unclear. The Nutzotin basins (fig. 1) where the stratigraphy is more difficult to

1Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397. Email for J.D. Manuszak: [email protected]

63 64 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY delineate because of metamorphism and deformation. Alaska Yukon GEOLOGIC SETTING Denali fault N Kahiltna Basin The Nutzotin Mountains sequence outcrops in the Nutzotin and Mentasta mountains of the eastern Alaska Range, southeastern central Alaska (fig. 2). The Denali fault separates the Nutzotin Basin Dezadeash Basin Nutzotin Mountains sequence from the parautochthonous Yukon–Tanana terrane to Gravina Basin the north (fig. 2). The Yukon–Tanana terrane Upper Jurassic- Lower Cretaceous basins Bowser Basin represents the dissected continental margin of North America (Richter and Jones 1973b, Wrangellia composite terrane British Hansen, 1990). Rocks of the Yukon–Tanana Columbia terrane consist of sparse orthogneiss of Late Late Jurassic-Tertiary accretionary complex Proterozoic age; polymetamorphosed and Tyaughton Basin polydeformed quartz-rich schist; pelitic schist; Coast Mountains batholith sparse marble of Precambrian, Devonian, and Mississippian age; and orthogneiss, augen Yukon- Tanana terrane gneiss, and intermediate to felsic volcanics of 0 400 Late Devonian to Early Mississippian age Fault Methow Basin (Plafker and Berg, 1994). km Political boundary U.S.A. The southern boundary of the Nutzotin Mountains sequence is the Wrangellia member of the Wrangellia composite terrane (fig. 2), an allochthonous island arc with deposits that are up to 3,000 m thick (Berg and others, 1972). respect to western North America (Jones and Crosscutting relationships with mid-Cretaceous plutons constrain others, 1986; Nokleberg and others, 1994; the age of the Chisana Formation to post-Valanginian (Richter and Plafker and Berg, 1994). The stratigraphy of Jones, 1973b). Berg and others (1972) note that the occurrence of the Wrangellia terrane near the study area the ammonite Shasticrioceras is indicative of a Barremian age includes: Pennsylvanian and Permian marine (Early Cretaceous) for the Chisana Formation. Most of the volcanic rocks of the Tetelna Formation; volcaniclastic strata are interpreted to be deposited by massive Lower Pennsylvanian and Lower Permian submarine lahars and debris flows (Richter and Jones, 1973b). The marine volcanic and sedimentary rocks of the volcanic flows are of transitional tholeiitic–calc–alkaline affinity Slana Spur and Station Creek Formations, (Barker, 1987). Unconformably overlying the Nutzotin Mountains Permian nonvolcanogenic limestone and sequence and the Chisana Formation are nonmarine argillite of the Eagle Creek and Hasen Creek conglomerates, sandstones, shales, and coals (Richter, 1971b; Formations; Upper Triassic submarine and Richter and Jones, 1973). Abundant well-preserved floras suggest subaerial basalts and mafic intrusives of the a Late Cretaceous age for these deposits (Richter and Jones, 1973). Nikolai Greenstone; and calcareous sediment- Within and locally along the southern margin of the Nutzotin ary rocks and limestones of the Chitistone, Mountains sequence, the Totschunda fault extends in a Nizina, and McCarthy limestones (Nokleberg northwestern direction from Canada to its junction with the Denali and others, 1994; Richter and Jones, 1973b; fault (fig. 2). Displacement is dominantly dextral slip with offsets Plafker and Berg, 1994). The Nutzotin of as much as 4 km (Plafker and others, 1977). Initiation of Mountains sequence lies both depositionally faulting may have begun as late as 200–400 ka based on a slip rate (Berg and others, 1972; Richter and Jones, of 1–2 cm/year (Plafker and others, 1977). The Denali fault, which 1973b) and in fault contact (Manuszak and borders the Nutzotin basin to the north, has a much longer history others, 1998) with the Wrangellia composite of displacement (Lanphere, 1977; Csejtey and others, 1982). terrane. Eisbacher (1976) and Lowey (1998) estimate 300–400 km of Depositionally overlying the Nutzotin right-lateral displacement on the Denali fault based on the Mountains sequence is the Chisana Formation interpretation that the Nutzotin Mountains sequence is the offset (fig. 2) which consists primarily of inter- equivalent of the Dezadeash basin found in northern Canada layered andesitic flows and volcaniclastic (fig. 1). STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 65

Figure 1 (left). Generalized tectonic map showing the distribution of Upper Jurassic to Lower Cretaceous basins and surrounding terranes along the western margin of North America (modified from McClelland and others, 1992). The Upper Jurassic to Lower Cretaceous basins (black) include marine clastic strata and volcanic rocks. The Wrangellia composite terrane (diagonal dashed pattern) includes the Peninsular, Alexander, and Wrangellia terranes. The Coast Mountains batholith (cross pattern) includes the Skagit terrane. The Nutzotin basin is located along the Canada–Alaska border in the north-central part of the diagram. Area of geologic map shown on figure 2 is outlined by the dashed rectangle surrounding the Nutzotin basin. Figure 2 (below). Generalized geologic map of the Nutzotin Mountains sequence within the Nabesna Quadrangle. Measured section locations are labeled and correspond to those found on figure 3. Geology is modified from Richter (1976). Scale = 1:250,000.

63° YTT YTT

KSC1 WCT SC1 BR1 YTT YTT AC1 SN1 TC1 r e LC1 v LR1 Ri Co na pper s Ri NC1 ver abe KCR1 N

r e iv R D a Totschunda enali fault n a is h C Nabesna F KJs ault MM1 MC1&2 CP1 N

KNC1&2 WCT CG1 GC1 KOZ BC1 KGC1&2 0km50 Chisana 62° 144° 141° EXPLANATION Qa: Quaternary deposits QTv: Quaternary/Tertiary volcanic rocks Fault Cretaceous Chisana volcanic rocks Inferred fault Cretaceous plutonic rocks Contact KJs: Nutzotin Mountains sequence River Facies Association 1 Facies Association 2 LC1 Location and name of measured section within the Nutzotin Mountains Facies Association 3 sequence Tertiary-Cretaceous sandstone and conglomerate Town WCT: Wrangellia composite terrane YTT: Yukon-Tanana terrane 66 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY

PREVIOUS RESEARCH 6–29 cm. Outsized limestone clasts, exceeding 10 m in diameter (figs. 4A and 4B), are observed in measured Previous studies of the Nutzotin Mountains sequence section MM1 and south of Nabesna. consist primarily of the initial regional geologic mapping Conglomerates in the upper part of the first litho- (Richter, 1971; Richter and Schmoll, 1973; Richter, 1976 facies have similar channelized geometries, but more Lowe and others, 1982). These initial investigations commonly are clast-supported, have smaller average divide the Nutzotin Mountains sequence into three maximum particle sizes (6–8 cm), and contain more mappable units: “upper,” “middle,” and “lower” interbedded shale and sandstone than the lower (Richter, 1976). We use these divisions as a starting point conglomerates. The upper conglomerates also contain a to construct the stratigraphic framework for the Nutzotin wider range of clast types that include tuff, quartz, Mountains sequence. greenstone, limestone, plutonic rocks, argillite, and chert. Previous tectonic interpretations for the formation of Paleocurrent data collected from imbricated clasts within the Nutzotin basin include: (1) deposition in a rapidly the upper conglomerates indicate northwestward subsiding long-linear-interarc (back-arc) basin within an paleoflow (measured section MM1; fig. 3). ancient volcanoplutonic arc (Berg and others, 1972); The black shale and sandstone found between and (2) deposition within a back-arc or intra-arc basin that commonly interbedded with the upper and lower formed on top of the Wrangellia terrane (Kozinski, conglomerates comprise 50 percent of the first lithofacies 1985); (3) “...sedimentation in large linear basins of FA1. The shale has horizontal laminations, but more generally far behind the continental margin” (Richter and commonly no structures are observed because of Jones, 1973b); and (4) evolution within or proximal to a pervasive fractures. The black shale contains disarticu- regional, transtensional, basinal arc complex lated bivalves, Buchia, in measured section MC2 and (McClelland and others, 1992). Our working hypothesis carbonaceous plant debris at the top of measured section suggests that the Nutzotin basin formed as a flexural MM1 (fig. 3). Throughout measured sections MM1, foredeep in response to the collision of the Wrangellia MC1, and MC2, gray-green porphyritic sills and dikes composite terrane with the North American continental commonly intrude the shale. The interbedded sandstones margin (Manuszak and others, 1998). have bed thicknesses that range from a few centimeters to a meter. Sandstones commonly display normal graded ACIES ASSOCIATIONS bedding, and isolated pebble- to cobble-sized conglome- We have defined three facies associations (FA1, FA2, rate clasts. We interpret the first lithofacies of FA1 to and FA3) within the Nutzotin Mountains sequence based represent proximal sedimentation in a submarine fan on new stratigraphic data and previous work (Berg and system (Manuszak and others, 1998). others, 1972; Kozinski, 1985; Richter, 1976; and Richter BLACK SHALE AND RED, CALCAREOUS MUDSTONE and Jones, 1973b). The second lithofacies of FA1 is at least 300 m thick ACIES ASSOCIATION 1 (A1) and found in measured sections AC1, TC1, and LC1 (fig. 3). The strata is best exposed in measured section The stratigraphically lowest facies association LC1 located in the northwestern corner of the basin, (FA1) is divided into two separate lithofacies. The first along the headwaters of Lost Creek (fig. 2). The black unit is a conglomerate with interbedded black shale and gray-green sandstone. The second unit consists of Figure 3 (right). Simplified and correlated stratigraphic black shale with interbedded red, calcareous mud- sections from the Nutzotin Mountains sequence. Label stone, and subordinate amounts of conglomerate. at the top of each section corresponds to location on figure 2. Vertical placement of each section based on CONGLOMERATE AND INTERBEDDED SHALE correlation of facies associations described in the text The first lithofacies is up to 200 m thick and is best and direct projection along strike where possible. exposed in the MC1, MC2 and MM1 measured sections “??” = uncertain stratigraphic correlation. Hori- along the south-central margin of the basin, east of the zontal placement of each section is based on its town of Nabesna (figs. 2 and 3). The lowermost general location in figure 2. Measured section KOZ conglomerates in the first lithofacies have the following compiled from Kozinski (1985). Paleocurrent data characteristics: (1) average bed thickness of ~1–1.5 m; (arrows) are representative of numerous measure- (2) channelized geometries; (3) mainly limestone clasts ments taken from a series of beds within a particular with minor amounts of chert, argillite, and volcanic part of the section and indicate average flow direction. clasts; (4) both matrix- and clast-supported framework; Types of sedimentary structures measured for and (5) average maximum particle size ranging from paleocurrent data are discussed in text. STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 67

0m 1,000 m 2,000 m Nutzotin Mountains Sequence Chisana Formation Facies Association 1 Facies Association 2 Facies Association 3 S 1 KSC Northwest o nl thrust angle Low n=30 AC1/TC1/LC1 SC1 rdtoa contact Gradational n=19 n=31 LR1 SN1 n=57 n=56 rdtoa contact? Gradational rnelacmoieterrane composite Wrangellia BR1 10kilometers ~160 ?? ?? NC1/KCR1 MM1 nua unconformity Angular MC1&2 n=25 CP1 KNC1&2/CG1 rdtoa contact Gradational ?? KOZ n=57 ocncrocks Volcanic mudstone Fossiliferous Shale/argillite shale and siltstone Graded Sandstone Conglomerate aecretdata Paleocurrent BC1/GC1 Explanation ?? ?? Southeast KGC1&2

Penn.- Upper Jurassic Upper Jurassic - Lower Cretaceous Lower Cretaceous Triassic 68 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY shale, which comprises the majority of this facies cross-stratification in the coarser units show flow to the association, is commonly devoid of primary east and northeast (measured section SC1; fig. 3). sedimentary structures and lacks macrofauna (fig. 4C). The noncalcareous siltstones and shales contain Faint subhorizontal laminations, however, are observed horizontal laminations, horizontal and vertical burrows in a few of the coarser beds. Bioturbation is present a few millimeters in width, strictly horizontal burrows up throughout the black shale in the form of both horizontal to 2 cm in width, and rip-up clasts. The calcareous and vertical burrows that are less than a few millimeters siltstones commonly lack sedimentary structures and are in diameter. The red calcareous mudstone beds are up to highly fractured. The tops of the fine-grained beds are 30 cm thick (fig. 4C) and commonly display boudinage, often scoured into and/or form an abrupt contact with the lack internal sedimentary structure, and become less overlying coarser sandstone beds (fig. 4G). Locally, this common toward the top of the unit. Locally, channelized unit is metamorphosed to argillite. We interpret the conglomerates up to 10 m in width and a meter in shales, siltstones, and sandstones of FA2 to represent thickness occur within the thick section of shale. The sandy, low-density turbidite deposits of an outer conglomerates are mostly matrix supported and consist submarine fan environment (Manuszak and others, predominantly of limestone clasts. Dikes and sills 1998). commonly intrude the shale. We interpret this lithofacies of FA1 to represent distal sedimentation on a submarine CONGLOMERATE AND SANDSTONE fan system. The second lithofacies of FA2 is up to 300 m thick and is laterally continuous and interbedded with the first AGE CONSTRAINT lithofacies of FA2. This stratum was documented in The two lithofacies of FA1 correlate to map units Ja measured sections KNC1, KNC2, SC1, KSC1, and the (Richter and Schmoll, 1973), Jcs (Richter, 1971), and top of BR1 (fig. 3). The best exposure of the second “lower unit” (Richter, 1976). These map units are lithofacies of FA2 is along Suslota Creek in the constrained by paleontologic data to be Late Jurassic northwestern part of the basin (measured section SC1; (Tithonian–Oxfordian) in age (Richter and Schmoll, figs. 2, 3). 1973; and Richter, 1971; Richter and Jones, 1973b). The sandstones are structureless, medium- to very coarse-grained, commonly contain pebbles and rip-up ACIES ASSOCIATION 2 (A2) clasts, may display flute casts at the base of individual beds, and have an average bed thickness of 0.5–1 m FA2 is divided into two lithofacies. The first (fig. 4E). The conglomerates are predominantly clast lithofacies consists of alternating beds of laterally supported and poorly sorted (fig. 4F), but a few beds continuous shale, noncalcareous siltstone, and sandstone have matrix support. The clast-supported conglomerates with subordinate amounts of light gray, calcareous have an average maximum particle size ranging from 5– siltstone (fig. 4D). The second lithofacies consists of 20 cm, whereas the matrix-supported conglomerates massively bedded, laterally continuous, green sandstone have an average maximum particle size of ~5 cm. Both (fig. 4E), and poorly sorted channelized conglomerate matrix- and clast-supported conglomerates contain (fig. 4F). greenstone, limestone, plutonic, and black chert clasts. Paleocurrent data collected from imbricated clasts and INTERBEDDED SHALE, SILTSTONE, AND SANDSTONE ripple stratification show paleoflow predominantly to the This lithofacies of FA2 is the most common in the east and northeast (measured sections KSC1 and SC1; basin. It is over 1,000 m thick and found in measured fig. 3). We interpret this unit of FA2 to represent gravelly sections KSC1, SC1, BR1, NC1, KCR1, KNC1, KNC2, and sandy high-density turbidite deposits of proximal CP1, and CG1 (fig. 3). This lithofacies is well exposed and medial portions of a submarine fan environment throughout the basin, but is best exposed in measured (Manuszak and others, 1998). section KCR1 (fig. 3) in the central part of the basin (fig. 2). AGE CONSTRAINT The coarser beds are comprised predominantly of The two units of FA2 correlate to map units Jg medium- to fine-grained sandstone. Sedimentary (Richter and Jones, 1973), KJms (Richter, 1971), KJg features include normal grading (fig. 4G); ripple cross- (Richter and Schmoll, 1973), KJs (Lowe and others, stratification; horizontal stratification; fine, wispy mud 1982; Richter and others, 1973, Richter and others, laminations; and flute casts. Individual bed thickness 1976), KJgc (Richter and Schmoll, 1973), and “middle ranges from less than a centimeter to half a meter. Rip- unit” (Richter, 1976). The Jg and KJs units of Richter and up clasts and pebble inclusions are common at the base Jones (1973) and Richter and others (1973), respectively, of the sandstones. Paleocurrent data collected from ripple have been constrained with paleontologic data to be Late STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 69

Jurassic. The other units are loosely constrained by are constrained by paleontologic data to be Late Jurassic paleontologic data to be Late Jurassic (Tithonian) to (Tithonian) to Early Cretaceous (Valanginian) in age. Early Cretaceous (Valanginian) in age (Richter, 1976). STRATIGRAPHIC CORRELATIONS ACIES ASSOCIATION 3 (A3) Twenty-one stratigraphic sections have been FA3 consists of at least 300 m of fossiliferous simplified and correlated on figure 3 to show our mudstone with distinctive fossil-rich horizons (fig. 4H), interpretation of the stratigraphic architecture of the minor interbedded sandstone, and andesitic flows Nutzotin Mountains sequence. The stratigraphic position (measured sections BC1, GC1, and KOZ; fig. 3). FA3 is of each measured section is based on correlation of facies best exposed at measured section BC1, located along associations and direct projection along strike where Bonanza Creek, in the southeastern corner of the basin possible. This type of correlation permits only a rough (fig. 2). approximation of the thickness for each facies The fossiliferous mudstone is commonly poorly association. Additional structural mapping and measured exposed, but abundant disarticulated and articulated sections are required in several areas to allow for a better Buchia, a bivalve, are observed. The mudstones also correlation of stratigraphic data. contain dewatering structures, laminations, rip-up clasts, and horizontal and vertical burrows. The distinctive AGE RELATIONSHIPS BETWEEN ACIES fossil-rich horizons occur as both coquina (figs. 4I, 4J) ASSOCIATIONS and in-situ deposits of Buchia. The in-situ horizons consist of densely clustered articulated bivalve shells that The correlation diagram shown on figure 3 provides are uniform in length (~4 cm down the long axis of the a general temporal relationship for the three facies shell), and form 10–20-cm-thick beds that are laterally associations. The age range from Oxfordian (Late continuous at the outcrop scale. The coquina beds consist Jurassic) to Barremian (Early Cretaceous) is based on of fragmented shell hash that is fairly uniform in size. sparse and poorly preserved species of Buchia (Richter, The beds range in thickness from a few centimeters to 1976; Richter, 1971; Richter, 1975; Richter and Jones, 0.75 m. Thin, laterally continuous coquina beds are 1973; Richter and Jones, 1973b; and Berg and others, interbedded with the in-situ fossil horizons. Thicker 1972). The broad and overlapping age ranges of Buchia coquina beds range in thickness from 0.25–0.75 m and in the Nutzotin basin suggest that, at times, the three are channelized over a distance of 5 m. The channelized facies associations were probably coeval, laterally coquinas are generally structureless, but horizontal equivalent lithofacies. Samples collected for palynologic stratification was observed in a few channel tops. The analysis and radiometric age determinations during the channelized coquinas grade upward into fossiliferous 1998 field season will hopefully permit more accurate mudstone and contain mudstone rip-up clasts up to 30 cm reconstruction of temporal and spatial relationships. in length (fig. 4J). Internally, the rip-up clasts contain soft sediment deformation. Clasts of tuff, coalified wood ACIES ASSOCIATION CONTACTS fragments, and isolated pebbles are also observed in FA3 (measured section GC1; fig. 3). CONTACT BETWEEN WRANGELLIA AND A1 The intercalated andesitic flows increase in We have recognized a potentially basin-wide, low- abundance up-section within FA3. The green sandstones, angle thrust fault that separates much of the Nutzotin located lower in FA3, are texturally and compositionally Mountains sequence from the underlying Wrangellia immature, and contain medium to coarse grains of composite terrane (Manuszak and others, 1998). This sedimentary and volcanic lithics, shell fragments, relationship is well exposed in the northwestern Mentasta feldspar, and quartz. The green sandstones range in Mountains at measured sections AC1, TC1, and LC1 (figs. thickness from less than a centimeter to 30 cm. We 2, 3). At measured section MC1, near the headwaters of interpret FA3 to represent the last stages of sedimentary the Nabesna River (fig. 2), the contact between FA1 and deposition in the Nutzotin basin before the onset of Wrangellia is an erosional unconformity (fig. 3). A similar Chisana volcanism. This facies association was probably depositional relationship can be seen directly south of deposited on a muddy, fauna-rich shelf that flanked the Nabesna. embryonic Chisana volcanic arc that was building in the southeastern corner of the basin. CONTACT BETWEEN A1 AND A2 Measured sections LR1 and SN1 (fig. 3) document AGE CONSTRAINT a gradational contact between FA1 and FA2 in the FA3 correlates to map units KJa (Richter and Jones, northeastern central part of the basin (fig. 2). The 1973) and “upper unit” (Richter, 1971). These map units gradational contact is characterized by a gradual loss of 70 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY

Figure 4A. Outsized limestone clast that is over 15 m in diameter found east of Nabesna at the base of Facies Association 1 (arrows point toward clast). Clast is surrounded by con- glomerate and interbedded shale.

Figure 4B. Limestone megaclast located south of Nabesna in Facies Association 1. Person (black arrow) is standing on the gray limestone clast. Notice the interbedded shale (white arrow) and limestone conglomerates surrounding the megaclast.

Figure 4C. Outcrop of the fine-grained lithofacies of Facies Association 1 displaying interbed- ded, red calcareous mudstone. Arrow points to 30-cm-long hammer whose head is resting on the top of a calcareous mudstone bed. STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 71

Figure 4D. Outcrop of rhythmically interbedded siltstones, sandstones, and shales of Facies Association 2. This is the dominant lithofacies of the Nutzotin Mountains sequence. Indi- vidual bed thickness within photo ranges from 5 to 20 cm. Bedding is dipping to the left. Arrow points to hammer (30 cm) for scale.

Figure 4E. Outcrop of laterally continuous, thick- bedded, tabular, green sandstones of Facies Association 2. Bedding dips to the right in the photograph, which is looking along strike. Hammer for scale.

Figure 4F. Close-up of poorly sorted conglome- rate of Facies Association 2. White arrow points to large clast of granodiorite. Black arrow points to a clast of limestone. Scale in upper right is 8 cm long. 72 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY

Figure 4G. Close-up photo of graded beds within Facies Association 2 that show ripple cross- lamination (arrow) within a medium-grained sandstone, and normal grading. Notice sharp contacts of sandstone bases (s), which grade upward into mudstone (m). Coin for scale.

Figure 4H. Outcrop of Facies Association 3 showing interbedded sandstone and fossiliferous shale. White arrows point to individual sandstones.

Figure 4I. Coquina lithofacies of Facies Association 3 composed of bivalve fragments capped by a laminated grainstone. Coin for scale. STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 73

Figure 4J. Close-up photo of fossil hash and mudstone rip-up clasts within a channel of Facies Association 3. Fossil hash is com- posed of disarticulated bivalve fragments. Arrows point to individual rip-up clasts. Scale is 8 cm long.

red calcareous siltstone and a corresponding increase in because of the presence of several faults of interbedded noncalcareous siltstone and shale. unknown displacement. The two lithofacies have similar ages (Richter, 1971b; Richter and Jones, CONTACT BETWEEN A2 AND A3 1973); therefore we interpret the south-central Our measured sections from the 1998 field season did not outcrops of FA1 to represent proximal deposits of include exposures that record the transition from FA2 to FA3. a submarine fan system. Paleocurrent data from A previous measured stratigraphic section from the southeastern these outcrops indicate northwestward sediment corner of the basin by Kozinski (1985), however, includes this transport within the fan system. The finer-grained contact (section KOZ; fig. 3). The gradational contact is marked outcrops of FA1 exposed in the northwestern part by a decrease in bed thickness, and increase in grain size of the basin are interpreted as more distal deposits (Kozinski, 1985). This contact is exposed along Bonanza Creek, of the same general submarine fan system. southwest of the town of Chisana (fig. 2). Facies Association 2 is extremely thick (>1,000 m) and is common throughout the entire CONTACT BETWEEN A2/A3 AND THE CHISANA ORMATION Nutzotin basin (fig. 2). The bulk of the coarse- Measured sections KOZ and BC1 document a gradational grained deposits of Facies Association 2 are contact between FA3 and the Chisana Formation (fig. 3). The concentrated in the northwestern segment of the contact is characterized by an upsection increase in intercalated basin (measured sections SC1 and KSC1; fig. 3). volcanic flows. In the southeastern part of the basin a gradational To the southeast along the axis of the basin, there contact between FA2 and the Chisana Formation exists at is a decrease in conglomerate and sandstone of measured section KGC1 and KGC2 (figs. 2, 3). The absence of FA2 and a corresponding increase in mudstone. FA3 in this part of the basin, and the gradational contact of both The reduction in grain size along the axis of the FA2 and FA3 with the Chisana Formation suggest that either basin, combined with paleocurrent data that FA3 was only deposited locally and pinches out to the southeast, indicate eastward paleoflow, suggest to us that and/or that FA2 and FA3 are spatially related coeval deposits. FA2 represents a major axial submarine fan Continued fieldwork during the 1999 field season will address system that was responsible for filling a large part this specific issue. of the Nutzotin basin. Gravelly and sandy high- density turbidite deposits of FA2 from the IMPLICATIONS O THE DISTRIBUTION O ACIES northwestern part of the basin represent proximal ASSOCIATIONS and medial parts of the submarine fan system. Silty and muddy low-density turbidite deposits of Facies Association 1 is restricted to the northwestern and the central and southeastern parts of the basin central southern margin of the Nutzotin basin (fig. 2). The represent the distal parts of the submarine fan northwestern outcrops of Facies Association 1 consist of system. mudstone with localized lenticular conglomerate channels. FA1 Facies Association 3 is restricted to a local exposed along the central southern basin margin consists of area within the southeastern part of the Nutzotin conglomerate containing marine macrofossils, coalified plant basin (fig. 2). The abundance of in-place and remains, and outsized limestone clasts. The lateral relationship disarticulated fossils, occurrence of coalified between the two areas cannot be established by direct correlation plant debris, and channelized coquina beds 74 SHORT NOTES ON ALASKA GEOLOGY 1999 MANUSZAK AND RIDGWAY

indicate a shallower marine environment of deposition Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, than for FA1 and FA2. The upsection increase in Cordilleran suspect terranes: Nature, v. 288, p. 329- intercalated volcanic flows suggests to us that FA3 is 333. closely related to the overlying Chisana Formation. We Csejtey, B., Jr., Cox, D.P., Evarts, R.C., Stricker, G.D., interpret FA3 to represent the last stages of sedimentary and Foster, H.L., 1982, The Cenozoic Denali fault deposition in the Nutzotin basin before the onset of system and the accretionary development of southern widespread Chisana volcanism. This facies association Alaska: Journal of Geophysical Research, v. 87, was probably deposited on a shallow muddy shelf, rich p. 3,741-3,754. in marine invertebrates, that flanked the growing Chisana Eisbacher, G.H., 1976, Sedimentology of the Dezadeash volcanic arc in the southeastern corner of the Nutzotin flysch and its implications for strike-slip faulting basin. along the Denali fault, Yukon Territory and Alaska: Canadian Journal of Earth Sciences, v. 13, p. 1,495- ACKNOWLEDGMENTS 1,513. Hansen V.L., 1990, Yukon–Tanana terrane: A partial We thank the Wrangell–St. Elias National Park and acquittal: Geology, v. 18, p. 365–369. Preserve staff, especially Danny Rosenkrans, for their Jones, D.L., Silberling, N.J., and Coney, P.J., 1986, help and support of our field activities. We also thank Collisional tectonics in the Cordillera of western Don Richter for encouragement and discussions on the North America: examples from Alaska, in Coward, Nutzotin Mountains sequence. Brian Lareau and Mike M.P., and Ries, A.C., eds., Collisional Tectonics: De Persia provided assistance in our field operations. London, United Kingdom, Geological Society of Acknowledgment is made to the donors of the Petroleum London Special Publication, v. 19, p. 367-387. Research Fund, administered by the American Chemical Kapp, P.A., and Gehrels, G.E., 1998, Detrital zircon Society, for support of this research. Partial support for constraints on the tectonic evolution of the Gravina this research was provided by the Geological Society of belt, southeastern Alaska: Canadian Journal of Earth America’s John T. Dillon grant, American Association of Sciences, v. 35, p. 253–268. Petroleum Geologist’s Fred A. Dix grant, Purdue Kozinski, J., 1985, Sedimentology and tectonic University’s Linda Horn Memorial Scholarship and significance of the Nutzotin Mountains sequence, Andrews Fellowship, Sigma Xi’s Alexander Bache Alaska: Albany, New York, State University of New Fund, and the Society of Petroleum Engineers. We thank York at Albany, Master’s Thesis, 132 p. reviewers George Gehrels and Don Richter for Lanphere, M.A., 1978, Displacement history of the improving this manuscript. Denali fault system, Alaska and Canada: Canadian Journal of Earth Sciences, v. 15, p. 817–822. REERENCES Lundberg, N., and Dorsey, R.J., 1988, Synorogenic Barker, Fred, 1987, Cretaceous Chisana island arc of sedimentation and subsidence in a Plio-Pleistocene Wrangellia, eastern Alaska: Geological Society of collisional basin, eastern Taiwan. in Kleinspehn, America Abstracts with Programs, v. 19, p. 580. K.L., and Paola, C., eds., New Perspectives in Basin Berg, H.C., Jones D.L., and Richter, D.H., 1972, Analysis: New York, Springer-Verlag, p. 265–280. Gravina–Nutzotin belt—tectonic significance of an Lowe, P.C., Richter, D.H., Smith, R.L., and Schmoll, upper Mesozoic sedimentary and volcanic sequence H.R., 1982, Geologic map of the Nabesna B-5 in southern and southeastern Alaska: U.S. Geological quadrangle, Alaska: U.S. Geological Survey Map Survey Professional Paper 800-D, p. D1–D24. GQ-1566, 1 sheet, scale 1:63,360. Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Lowey, G.W., 1998, A new estimate of the amount of Tectonic overview of the Cordilleran orogen in the displacement on the Denali fault system based on the western United States, in Burchfiel, B.C., Lipman, occurrence of carbonate megaboulders in the P.W., and Zoback, M.L., eds, The Geology of North Dezadeash Formation (Jura-Cretaceous), Yukon, and America—The Cordilleran Orogen, Contermin- the Nutzotin Mountains Sequence (Jura-Cretaceous), ous U.S.: Geological Society of America, v. G-3, Alaska: Bulletin of Canadian Petroleum Geology, p. 407-479. v. 46, p. 379–386. Cohen, H.A., Hall, C.M., and Lundberg, Neil, 1995, Manuszak, J.D., Ridgway, K.D., and Lareau, B.L., 1998, 40Ar/39Ar dating of detrital grains constrains the Stratigraphic architecture of a collisional basin, provenance and stratigraphy of the Gravina belt, Nutzotin Mountains sequence, Alaska Range, south southeastern Alaska: Journal of Geology, v. 103, central Alaska: Geological Society of America p. 327-337. Abstracts with Programs, v. 30, p. 338. STRATIGRAPHIC ARCHITECTURE O THE UPPER JURASSIC–LOWER CRETACEOUS NUTZOTIN MOUNTAINS SEQUENCE 75

McClelland, W.C., Gehrels, G.E., and Saleeby, J.B., Richter, D.H., 1975, Reconnaissance geologic map of the 1992, Upper Jurassic–Lower Cretaceous basinal Nabesna B-3 quadrangle, Alaska: U.S. Geological strata along the Cordilleran margin: implications for Survey Map I-904, scale 1:63,360. the accretionary history of the Alexander– ———1976, Geologic map of the Nabesna quadrangle, Wrangellia–Peninsular Terrane: Tectonics, v. 11, Alaska: U.S. Geological Survey Map p. 823–835. I-932, scale 1:250,000. Nokleberg, W.J., Plafker, George, and Wilson, F.H., Richter, D.H., and Jones, D.L., 1973, Reconnaissance 1994, Geology of south-central Alaska, in Plafker, geologic map of the Nabesna A-2 quadrangle, George, and Berg, H.C., eds., The Geology of Alaska: U.S. Geological Survey Map I-749, scale Alaska: The Geology of North America, Geological 1:63,360. Society of America, v. G-1, p. 311–366. Richter, D.H., and Jones, D.L., 1973b, Structure and Plafker, George, and Berg, H.C., 1994, Overview of the stratigraphy of eastern Alaska Range, Alaska: Tulsa, geology and tectonic evolution of Alaska, in Plafker, Oklahoma, American Association of Petroleum George, and Berg, H.C, eds., The Geology of Alaska: Geologists Memoir, v. 19, p. 408–420. The Geology of North America, Geological Society Richter, D.H., Matson, N.A., Jr., and Schmoll, H.R., of America, v. G-1, p. 989–1,021. 1973, Geologic map of the Nabesna A-1 quadrangle, Plafker, George, Hudson, T., and Richter, D.H., 1977, Alaska: U.S. Geological Survey Map I-807, scale Preliminary observations on late Cenozoic dis- 1:63,360. placements along the Totschunda and Denali fault ———1976, Geologic map of the Nabesna C-4 systems: U.S. Geological Survey Circular 751-b, quadrangle, Alaska: U.S. Geological Survey Map p. B67–B69. GQ-1303, scale 1:63,360. Richter, D.H., 1971, Reconnaissance geologic map and Richter, D.H., and Schmoll, H.R., 1973, Geologic map section of the Nabesna A-3 quadrangle, Alaska: U.S. of the Nabesna C-5 quadrangle, Alaska: U.S. Geological Survey Map I-655, scale 1:63,360. Geological Survey Map GQ-1062, scale 1:63,360. ———1971b, Reconnaissance geologic map and Rubin, C.M., and Saleeby, J.B., 1991, The Gravina section of the Nabesna B-4 quadrangle, Alaska: U.S. sequence: Remnants of a mid-Mesozoic oceanic arc Geological Survey Map I-656, scale 1:63,360. in southern Southeast Alaska: Journal of Geophysical Research, v. 96, p. 14,551–14,568.

STRATIGRAPHY, DEPOSITIONAL SYSTEMS, AND AGE O THE TERTIARY WHITE MOUNTAIN BASIN, DENALI AULT SYSTEM, SOUTHWESTERN ALASKA1 Kenneth D. Ridgway,2 Jeffrey M. Trop,2,3 and Arthur R. Sweet4

ABSTRACT

The White Mountain strike-slip sedimentary basin is located adjacent to the Farewell segment of the Denali fault system in southwestern Alaska. A detailed stratigraphic section from the basin documents three main lithofacies. The lithofacies, in order of abundance, include (1) granule and pebble conglomerate; (2) massive and trough cross-stratified sandstone; and (3) mudstone. The conglomerates and sandstones contain evidence of stream-flow processes such as clast support, imbrication, crude upward-fining trends, trough cross- stratification, and unimodal paleocurrent indicators. Paleocurrent data indicate that the exposed strata of the White Mountain basin were deposited predominantly by west-flowing fluvial systems. Palynological analyses indicate that deposition in the White Mountain basin occurred during the late Oligocene and possibly into the earliest Miocene. We interpret the White Mountain basin to be time-correlative to other Oligocene strike- slip basins along the eastern and central parts of the Denali fault system such as the McGrath, Talkeetna, Burwash, and Bates Lake basins. Our correlation suggests that the Oligocene was an important time for strike- slip displacement and basin development along much of the ~2,000 km length of the fault system.

INTRODUCTION

The Denali fault system is one of the major fault south side of the Denali fault system in southwestern systems of the northern Cordillera, extending more than Alaska, are offset equivalents to strata of the lower 2,000 km from British Columbia to southwestern Alaska Cantwell Formation (Late Cretaceous) in the Cantwell (fig. 1; Grantz, 1966; Lanphere, 1978; Dodds, 1995). basin, exposed on the north side of the Denali fault in the Although previous studies have demonstrated that strike- central Alaska Range (fig. 1). Sainsbury (1965) was the slip displacement along the fault system is probably post- first to tentatively interpret the White Mountain basin Early Cretaceous (Dodds, 1995), a more precise age of deposits as time-correlative to the lower Cantwell displacement is lacking. The amount of offset along the Formation. His correlation, however, was based solely Denali fault system is also controversial, with estimates on lithologic similarities. Our study uses palynological ranging from 0 to 450 km (see Lowey, 1998, for review). analysis to determine if the White Mountain basin One potential data set that has been underutilized in deposits are time-correlative with the lower Cantwell analyzing the history of the Denali fault system is the Formation. A recent palynological analysis of the lower Mesozoic and Cenozoic sedimentary basins that outcrop Cantwell Formation has shown it to be Late Cretaceous adjacent to the fault system (for example, Eisbacher, (Ridgway and others, 1997) but a similar analysis has not 1976; Nokleberg and others, 1985; Cole and others, been reported from strata of the White Mountain basin. 1996, 1999; Ridgway and others, 1997; fig. 1). A Our study also presents a measured stratigraphic section regional stratigraphic and geochronologic database of all from the White Mountain basin for comparison with the the basins along the fault system has the potential to stratigraphy of the Cantwell basin (Trop, 1996; Ridgway better define timing of displacement, and to determine if and others, 1997). Stratigraphic data were collected to individual basins have been offset by the fault and can determine if the two basins had similar depositional serve as markers to calculate fault displacement. This histories and were possibly offset-equivalents. If the paper presents the first detailed stratigraphic, basins are offset-equivalents, about 350 km of dextral sedimentologic, and geochronologic data from displacement would be required along the western part sedimentary strata that crop out for over 15 km along the of the Denali fault system, a value similar to estimates of Farewell segment of the Denali fault system in offset along the eastern part of the fault system southwestern Alaska. These strata are informally named (Eisbacher, 1976; Plafker and Berg, 1994). the White Mountain basin (figs. 1, 2). Another previous mapping study (Gilbert, 1981) One catalyst for this study was to test a hypothesis assigned a Tertiary age to the White Mountain basin that strata of the White Mountain basin, exposed on the deposits because of their lithologic similarity to the

1Geological Survey of Canada Contribution No. 1998218. 2Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397. Email for Kenneth D. Ridgway: [email protected] 3Present address: Department of Geology, Bucknell University, Lewisburg, PA 17837. 4Geological Survey of Canada, Calgary, Alberta T2L 2A7, Canada.

77 78 SHORT NOTES ON ALASKA GEOLOGY 1999 RIDGWAY AND OTHERS

Usibelli Group (Wahrhaftig and others, 1969; Wahr- others, 1998, 1999) of southern Alaska and northwestern haftig, 1987). The Usibelli Group is exposed north of the Canada. central Alaska Range in the Usibelli basin (fig. 1). The Usibelli Group ranges in age from Late Eocene to STRATIGRAPHY AND DEPOSITIONAL SYSTEMS Miocene, but most of the deposits are Miocene (Wahrhaftig and others, 1969; Leopold and Liu, 1994). At Our stratigraphic and sedimentologic data from the the type section, for example, the entire Usibelli Group White Mountain basin are from a 440 m detailed (about 585 m) is Miocene (Leopold and Liu, 1994). measured stratigraphic section (fig. 3) located in the This paper presents new geochronologic, northeastern corner of the basin (fig. 2). The basal sedimentologic, and paleocurrent data from surface contact between deposits of the White Mountain basin exposures of the White Mountain basin. Our analysis and underlying rocks is not exposed, and the top of the allows comparison of timing of deposition in the White section is eroded; therefore the original thickness of the Mountain basin to Late Cretaceous development of thrust- basin fill cannot be determined. The measured section at top basins (Cantwell basin on fig. 1; Ridgway and others, White Mountain is dominated by three main lithofacies, 1997), Eocene-Oligocene strike-slip basins (Burwash and which are, in order of abundance: (1) granule and pebble Bates Lake basins; Ridgway and DeCelles, 1993a, b), and conglomerate; (2) massive and trough cross-stratified Miocene foreland basins (Usibelli Group; Ridgway and sandstone; and (3) mudstone.

70û 164û 164û 156û 148û 140û 132û 148û

0 300 km

USSR

UNITED STATES

C

UNITED STATES ALASKA A

N

A

D YUKON A TERRITORY

Kaltag fault Usibelli basin Tintina fault Cantwell basin 62û Talkeetna basin Denali McGrath basin fault White Mountain Burwash basin basin Area of Sheep Creek basin figure 2 Bates Lake basin

Three Guardsmen basin BRITISH Gulf of COLUMBIA Alaska

egathrust m

PACIFIC OCEAN leutian 54û A

Figure 1. Map of the Denali fault system in British Columbia, Yukon Territory, and Alaska showing the location of sedimentary basins (dark gray shaded areas). The Sheep Creek and Three Guardsmen strike-slip basins are not discussed in the text, but their locations are shown. TERTIARY WHITE MOUNTAIN BASIN, DENALI AULT SYSTEM, SOUTHWESTERN ALASKA 79

LITHOACIES 1 – GRANULE AND PEBBLE CONGLOMERATE

Clast-supported, granule and pebble conglomerate is the dominant lithofacies in the White Mountain basin measured section (figs. 3, 4). Conglomerates are medium- to thick-bedded, are well sorted, and contain rounded clasts (fig. 5A). Average maximum clast size in the conglomerate is 5 cm (figs. 3, 5B). Common clast types in the conglomerates are quartz, argillite, and chert (fig. 3). Lenticular sandstones (less than 50 cm thick) are commonly interbedded within the conglomerates. Where Lithofacies 1 is best exposed in our measured section (for example, 12.5 to 18 m and 34 to 40 m on fig. 3), it consists of upward- Figure 2. Generalized fining sequences that have a lower 1- to geologic map of the White 4-m-thick conglomerate unit capped by a Mountain basin located in 0.5- to 1-m-thick massive sandstone. Few the McGrath Quadrangle of sedimentary structures were recognized in south-western Alaska. See figure 1 Lithofacies 1, but clast imbrication is present. At five for regional location of the basin. The different stratigraphic positions in the section, we Farewell fault is a segment of the Denali measured 10 imbricated clasts from a single fault system. PZu = Mesozoic–Paleozoic conglomerate bed to determine paleocurrent direction. rocks of the Farewell terrane and White The stratigraphic position of these measurements and Mountain sequence (Decker and others, restored paleocurrent orientations are shown on figure 3. 1994); Ts = sedimentary strata of the The measurements indicate an overall westward White Mountain basin; Qs = Quaternary paleoflow. deposits. MS = location of measured stratigraphic section shown on figure 3. LITHOACIES 2 – MASSIVE AND TROUGH Geology and structural data from Gilbert CROSS-STRATIIED SANDSTONE (1981).

Lithofacies 2 consists mainly of massive, Table 1. Type and abundances of pollen and miospores coarse- to medium-grained sandstone with maximum bed thicknesses of ~6 m. Trough Species Percentage of Sample cross-stratification was documented in some 155 m 330 m 387 m units but other sedimentary structures are Gymnosperm pollen uncommon. Conglomerate lags and lenses are T/C/Ta 65.2 83.1 42.0 common in Lithofacies 2. Bisaccate 6.5 3.1 21.0 Tsuga 3.3 0.8 2.1 LITHOACIES 3 – MUDSTONE Miospores Lithofacies 3 consists of poorly exposed Laevigatosporites 11.4 4.6 8.4 mudstone that was best studied by trenching. Lycopodium 0.0 0.0 0.4 Most of the lithofacies consists of alternating Cyathidites 0.0 0.0 0.4 thin beds (less than 15 cm) of siltstone and Angiosperm pollen shale. Plant fragments were documented in Alnus 8.2 5.8 19.3 this lithofacies. Samples for palynological Betulacae 3.3 2.3 5.5 analysis were collected from this lithofacies Pterocarya 0.0 0.0 0.4 (table 1). Tricolpate 0.0 0.0 0.4 Total Number of Taxa Counted 230 254 238 aT/C/T = Taxodiaceae/Cupressaceae/Taxaceae 80 SHORT NOTES ON ALASKA GEOLOGY 1999 RIDGWAY AND OTHERS

Figure 3. Measured stratigraphic section of the White Mountain basin showing lithofacies in the northeastern part of the basin. Vertical scale is in meters. Each arrow represents restored paleocurrent direction based on mean measurement of ten imbricated conglomerate clasts. See figure 2 for location of measured section. TERTIARY WHITE MOUNTAIN BASIN, DENALI AULT SYSTEM, SOUTHWESTERN ALASKA 81

Figure 4. Typical outcrops of the White Mountain basin. Beds are dipping about 45° southeast (to the left in the photo). Conglomerates and sandstones of Lithofacies 1 and 2 form resistant along-strike ridges. Recessive grassy layers are mudstones of Lithofacies 3. Trace of the Farewell fault is in the large valley between the outcrops and the man. Strati- graphic section shown in figure 3 was measured in this area.

Figure 5A. Granule and pebble conglomerate of Lithofacies 1. Lichens cover the lower right part of the outcrop. Hammer is 28 cm long.

Figure 5B. Close-up of clast- supported conglomerate of Lithofacies 1. Pen is about 14 cm long. 82 SHORT NOTES ON ALASKA GEOLOGY 1999 RIDGWAY AND OTHERS

DEPOSITIONAL SYSTEMS INTERPRETATION Quercus, and Tilia) further restricts the age to within the range of Oligocene to possibly early Pliocene. The We interpret the White Mountain basin strata as apparent absence of Parviprojectus makes it unlikely that having been deposited predominantly by fluvial the samples are of early Oligocene age. When compared depositional systems. The conglomerates (Lithofacies 1) to the late early Miocene Upper Ramparts Canyon and sandstones (Lithofacies 2) contain evidence of organic bed 1 in central Alaska, which is dominated by stream-flow processes including clast support, bisaccates with relatively low Taxodiaceae/Cupres- imbrication, crude upward-fining trends, trough cross- saceae/Taxaceae counts (White and Ager, 1994), our stratification, and fairly unimodal paleocurrent data does not compare well (table 1). Deposition during indicators. Lithofacies 1 and 2 appear similar to the late early Miocene is, therefore, unlikely (J.M. White, lithofacies described for modern low-sinuosity stream written commun., 1996). The low diversity and the lack systems where gravel and sand are transported as of thermophilous taxa in the White Mountain basin bedload and deposited on longitudinal bars and within samples (even in the three best preserved assemblages; channels (Rust, 1978; Collinson, 1986). The laterally table 1) eliminate the warm interval within the middle discontinuous lenses of sandstone within conglomerates Miocene (White and others, 1997). The lack of herbs of Lithofacies 1 were probably deposited in shallow (Cyperaceae, Ambrosia, Poaceae, other Asteraceae, channels on bar tops and along the flanks of bars during Polemoniaceae) probably eliminates the late Miocene– falling-stage and low-stage flows (Miall, 1977). Pleistocene (White and others, 1997). The mudstones of Lithofacies 3 require low energy In addition to the constraints stated above, the depositional processes with a large component of abundance of Picea and Tsuga, the dominance of Alnus suspension fallout. Suspension fallout within the in the angiosperm component, the less frequent presence proposed fluvial deposystem probably occurred mainly of Pterocarya, and the possibly rare occurrence of in overbank areas where flood water underwent a sudden Juglans, suggest that the late Oligocene or earliest decrease in velocity or where sediment entered ponded Miocene is the most probable age for the assemblage— areas within the basin. A fluvial overbank interpretation a time of relatively cool but moist climate (Wolfe, 1986; for Lithofacies 3 is consistent with the presence of plant J.M. White, written commun., 1996). Similar assem- megafossils, a lack of marine megafossils, and the blages are present in the youngest strata of the Oligocene absence of marine microfossils (checked for during Amphitheatre Formation, Burwash basin, Canada, but palynological analyses). they contain Parviprojectus (Ridgway and others, 1995). Our preferred age interpretation for the exposed strata of AGE the White Mountain basin, then, is late Oligocene possibly extending into the earliest Miocene. The age of the White Mountain basin deposits has been unclear due to a lack of age data. Previous studies DISCUSSION have interpreted the age of the White Mountain strata as Early Cretaceous (Sainsbury, 1965) or Tertiary (Gilbert, This study shows that the deposits of the White 1981) on the basis of regional lithologic correlations. To Mountain basin along the Farewell segment of the Denali better constrain the age of the White Mountain basin, fault system in southwestern Alaska were deposited by palynological analyses were conducted by one of us fluvial deposystems during the late Oligocene and (ARS) on 17 mudstone samples collected from our possibly into the earliest Miocene. The major fluvial measured stratigraphic section. Locations of samples are system probably entered the basin from the east and shown on figure 3. Table 1 shows counts from the three deposited sediment as it flowed westward along the axis best preserved assemblages recovered from the of the White Mountain basin. The late Oligocene or stratigraphic section (155, 330, and 387 m). Similar earliest Miocene fluvial system may have been part of a palynoflora are present in all 17 of the samples over the regional drainage system that flowed west- entire 440 m of measured section. southwestward, possibly in part along a trough Palynological analyses indicate that strata of the controlled by the Denali fault system. The same general White Mountain basin were most likely deposited during fluvial system may have deposited sediment in other the late Oligocene or earliest Miocene. The age nonmarine strike-slip basins presently located to the interpretation is based on the following arguments. The northeast along the Denali fault system, including the presence of Alnus, Pterocarya, and Tsuga, of modern McGrath and Talkeetna basins of Dickey (1984) (fig. 1). aspect, restricts the age to being within the range of The McGrath basin, located about 25 km northeast of the Eocene to Miocene or possibly early Pliocene (table 1). White Mountain basin, is characterized by an overall The persistent absence of Pistillipollenites and a wide southward paleoflow (Dickey, 1984). Dickey (1984) range of warm temperate taxa (Fagus, Liquidambar, reports that no palynomorphs were recovered from TERTIARY WHITE MOUNTAIN BASIN, DENALI AULT SYSTEM, SOUTHWESTERN ALASKA 83 samples collected during his study of the McGrath basin, REERENCES but that unpublished data of ARCO Alaska, Inc. identified Eocene to middle Oligocene pollen in Cole, R.B., Ridgway, K.D., Layer, P.W., and Drake, equivalent deposits. The Talkeetna basin was mapped by Jeffrey, 1996, Volcanic history, geochronology, and Reed and Nelson (1980) along the north side of the deformation of the upper Cantwell Formation, Denali central part of the Denali fault system (fig. 1). No National Park, Alaska: Early Eocene transition palynological studies have been published on the between terrane accretion and strike-slip tectonics: Talkeetna basin. Dickey (1984) correlated the Talkeetna Geological Society of America Abstracts with basin with the McGrath basin based on similar petrologic Programs, v. 28, p. 313. characteristics. ———1999, Kinematics of basin development during We interpret the White Mountain basin to be time- the transition from terrane accretion to strike-slip equivalent to Eocene-Oligocene strike-slip basins tectonics, Late Cretaeous–early Tertiary Cantwell located along the eastern part of the Denali fault system Formation, south central Alaska: Tectonics, v. 18, in the Yukon Territory, Canada, on the basis of similar p. 1224-1244. pollen assemblages (for example, Burwash and Bates Collinson, J.D., 1986, Alluvial sediments, in Reading, Lake basins on fig. 1). For a detailed discussion on the H.G., ed., Sedimentary environments and facies: palynology and structural development of strike-slip Oxford, United Kingdom, Blackwell, p. 20-62. basins in the Yukon Territory, see Ridgway (1992) and Decker, John, Bergman, S.C., Blodgett, R.B., Box, S.E., Ridgway and others (1995). Our correlation disagrees Bundtzen, T.K., Clough, J.G., Coonrad, W.L., with Sainsbury’s (1965) correlation of the White Gilbert, W.G., Miller, M.L., Murphy, J.M., Robinson, Mountain basin with the Late Cretaceous Cantwell basin M.S., Wallace, W.K., (1994), Geology of south- (fig. 1) and with Gilbert’s (1981) correlation of the White western Alaska, in Plafker, George, and Berg, H.C., Mountain basin with the Usibelli basin (fig. 1). Since eds., The Geology of Alaska: Boulder, Colorado, Gilbert’s (1981) map was published, detailed Geological Society of America, The Geology of palynological studies have shown that most of the North America, v. G-1, p. 285-310. Usibelli Group was deposited during the middle and late Dickey, D.B., 1984, Cenozoic non-marine sedimentary Miocene (Leopold and Liu, 1994; Liu and Leopold, rocks of the Farewell fault zone, McGrath 1994). If the White Mountain basin is time-correlative to Quadrangle, Alaska: Sedimentary Geology, v. 38, strike-slip basins along the eastern and central parts of p. 443-463. the Denali fault system, it would suggest that the Dodds, C.J., 1995, Denali fault system, in Gabrielse, H., Oligocene was an important time for strike-slip and Yorath, C.J., eds., Structural styles, Chapter 17: displacement and basin development along the entire Geology of the Cordilleran Orogen in Canada: fault system. Geological Survey of Canada, no. 4, p. 656-657. Our analysis of the White Mountain basin is limited Eisbacher, G.H., 1976, Sedimentology of the Dezadeash to one measured section, and much more research needs flysch and its implications for strike-slip faulting along to be done on this basin and other strike-slip basins along the Denali fault, Yukon Territory and Alaska: Cana- the Denali fault system. Not until all the basins along this dian Journal of Earth Sciences, v. 13, p. 1495-1513. fault system are studied will a complete understanding of Gilbert, W.G., 1981, Preliminary geologic map of the this ~2,000-km-long tectonic feature be possible. We Cheeneetnuk River area, Alaska: Alaska Division of hope that the data from this study will be a useful starting Geological & Geophysical Surveys, Alaska Open- point for future studies of the White Mountain basin. File 153, one sheet, scale 1:63,360, 10 p. Grantz, Arthur, 1966, Strike-slip faults in Alaska (Ph.D. ACKNOWLEDGMENTS thesis): Stanford University, California, 82 p. Lanphere, M.A., 1978, Displacement history of the This research was funded by grants from the National Denali fault system, Alaska and Canada: Canadian Science Foundation (EAR-9406078 and EAR- Journal of Earth Sciences, v. 15, p. 817-822. 9725587). We thank Robert Blodgett for making us Leopold, E.B., and Liu, Gengwu, 1994, A long pollen aware of the White Mountain basin, and for advice and sequence of Neogene age, Alaska Range: Quaternary airphotos needed to access the area. We also thank James International, v. 22/23, p. 103-140. White for useful discussions on the age of the pollen Liu, Gengwu, and Leopold, E.B., 1994, Climatic assemblages. Constructive reviews by Warren Nokle- comparison of Miocene pollen flora from northern berg, Rocky Reifenstuhl, and an anonymous reviewer East-China and south-central Alaska, USA: Palaeo- improved the manuscript. geography, Palaeoclimatology, Palaeoecology, v. 108, p. 217-228. 84 SHORT NOTES ON ALASKA GEOLOGY 1999 RIDGWAY AND OTHERS

Lowey, G.W., 1998, A new estimate of the amount of Ridgway, K.D., Trop, J.M., Nokleberg, W.J., and displacement on the Denali fault system based on the Davidson, C.M., 1998, Mesozoic and Cenozoic occurrence of carbonate megaboulders in the tectonics of the eastern and central Alaska Range: Dezadeash Formation (Jura-Cretaceous), Yukon, and Progressive basin development and deformation the Nutzotin Mountains sequence (Jura-Cretaceous), within a suture zone: Geological Society of America Alaska: Bulletin of Canadian Petroleum Geology, Abstracts with Programs, v. 30, p. A-242. v. 46, p. 379-386. Ridgway, K.D., Trop, J.M., and Jones, D.E., 1999, Miall, A.D., 1977, A review of the braided-river Petrology and provenance of the Neogene depositional environments: Earth Science Reviews, Usibelli Group and Nenana Gravel: Implications v. 13, p. 1-62. for the denudation history of the central Alaska Nokleberg, W.J., Jones, D.L., and Silberling, N.J., 1985, Range: Journal of Sedimentary Research, v. 69, p. Origin and tectonic evolution of the Maclaren and 1262-1275. Wrangellia terranes, eastern Alaska Range: Geologi- Rust, B.R., 1978, Depositional models for braided cal Society of America Bulletin, v. 96, p. 1251-1270. alluvium, in Miall, A.D., ed., Fluvial sedimentology: Plafker, George, and Berg, H.C., 1994, Overview of the Memoir, Canadian Society of Petroleum Geologists, geology and tectonic evolution of Alaska, in Plafker, Calgary, v. 5, p. 605-625. G., and Berg, H.C., eds., The Geology of Alaska: Sainsbury, C.L., 1965, Previously undescribed Middle(?) Geological Society of America, The Geology of Ordovician, Devonian(?), and Cretaceous(?) rocks, North America, v. G-1, p. 989-1021. White Mountain area, near McGrath, Alaska: U.S. Reed, B.L., and Nelson, S.W., 1980, Geologic map of the Geological Survey Professional Paper 525-C, p. C91- Talkeetna quadrangle, Alaska: U.S. Geological C95. Survey Miscellaneous Investigations Map I-1174A, Trop, J.M., 1996, Sedimentological, provenance, and 15 p., scale 1:250,000. structural analysis of the lower Cantwell Formation, Ridgway, K.D., 1992, Cenozoic tectonics of the Denali Cantwell basin, central Alaska Range: Implications fault system, Saint Elias Mountains, Yukon Territory: for thrust-top basin development along a suture zone Synorogenic sedimentation, basin development, and (unpublished M.S. thesis): West Lafayette, Indiana, deformation along a transform fault system (Ph.D. Purdue University, 222 p. thesis): Rochester, New York, University of Wahrhaftig, Clyde, 1987, The Cenozoic section at Rochester, 508 p. Suntrana Creek, in Hill, M.L., ed., Cordilleran Ridgway, K.D., and DeCelles, P.G., 1993a, Stream- section of the Geological Society of America: dominated alluvial-fan and lacustrine depositional Boulder, Colorado, Geological Society of America, systems in Cenozoic strike-slip basins, Denali fault Centennial Field Guide, v. 1, p. 445-450. system, Yukon Territory: Sedimentology, v. 40, Wahrhaftig, Clyde, Wolfe, J.A., Leopold, E.B., and p. 645-666. Lanphere, M.A., 1969, The Coal-Bearing Group in ———1993b, Petrology of mid-Cenozoic strike-slip the Nenana coal field, Alaska: U.S. Geological basins in an accretionary orogen, St. Elias Survey Bulletin 1274-D, 30 p. Mountains, Yukon Territory, Canada, in Johnsson, White, J.M., and Ager, T.A., 1994, Palynology, M.J., and Basu, Abhijit, eds., Processes controlling paleoclimatology, and correlation of Middle Miocene the composition of clastic sediments: Geological beds from the Porcupine River (Locality 90-1), Society of America Special Paper 284, p. 67-89. Alaska: Quaternary International, v. 22/23, p. 43-77. Ridgway, K.D., Sweet, A.R., and Cameron, A.R., 1995, White, J.M., Ager, T.A., Adam, D.P., Leopold, E.B., Liu, Climatically induced floristic changes across the Gengwu, Jetté, Helene, and Schweger, C.E., 1997, An Eocene-Oligocene transition in the northern high 18 million year record of vegetation and climate latitudes, Yukon Territory, Canada: Geological change in northwestern Canada and Alaska: Tectonic Society of America Bulletin, v. 107, p. 676-696. and global climatic correlates: Palaeogeography, Ridgway, K.D., Trop, J.M., and Sweet, A.R., 1997, Palaeoclimatology, Palaeoecology, v. 130, p. 293-306. Thrust-top basin formation along a suture zone, Wolfe, J.A., 1986, Tertiary floras and paleoclimates of Cantwell basin, Alaska Range: Implications for the Northern Hemisphere, in Broadhead, T.W., ed., development of the Denali fault system: Geological Land plants, notes for a short course: San Antonio, Society of America Bulletin, v. 109, p. 505-523. Texas, Paleontological Society, p. 182-225. LATE DEVONIAN (EARLY RASNIAN) CONODONTS ROM DENALI NATIONAL PARK, ALASKA Norman M. Savage,1 Robert B. Blodgett,2 and Phil . Brease3

INTRODUCTION

A brachiopod-bearing sample, collected in 1980 by At locality 1, the overlying limestone unit is at the Dr. Norman Silberling from dark gray, nodular base of a prominent, south-facing limestone cliff, about argillaceous limestone beds in the upper part of an 18.3 m (60 ft) below the ridge crest. Conodonts from the undated and unnamed shale unit in Denali National Park, upper limestone unit include Playfordia aff. P. primitiva was processed for conodonts by Savage but yielded only (Bischoff and Ziegler, 1957), Polygnathus cf. P. robustus a few broken specimens of general Middle to Late Klapper and Lane, 1985, Polygnathus webbi Stauffer, Devonian aspect. New samples were collected in 1994 1938, Ancyrodella pristina Khalymbadzha and by Csejtey (Csejtey and others, 1996; Dumoulin and Chernysheva, 1970, Mesotaxis cf. M. falsiovalis others, 1998), and by Blodgett and Brease from the site Sandberg, Ziegler, and Bultynck, 1989, Icriodus (locality 2 herein) and a nearby site (locality 1 herein) at symmetricus Branson and Mehl, 1934, and Mehlina sp. the base of an overlying, unnamed massive limestone The interesting feature of the conodont fauna is the unit. These two localities, each yielding conodonts and abundance of Playfordia aff. P. primitiva, which is the brachiopods, are located on the north side of a promi- most common species present. The presence of this nent saddle in NE¼ NE¼ sec. 30, T.17 N., R.12 W., species, and Mesotaxis cf. M. falsiovalis, indicates the Healy B-6 Quadrangle (fig. 1). transitans to punctata Zones of Ziegler and Sandberg

149°55’ C

A

To k la t R Healy iv e r Quadrangle

ALASKA 40 00

B Locality 1 .. Healy Locality 2

63°25’ 63°25’ Mt.Deborah

Cantwell

4500 Inset map 0 0 5000 5 6000 5 Denali

0 1km Scale Healy Quadrangle

149°55’

Figure 1. Locality maps for the Toklat River conodont samples. (A) Location of Healy 1:250,000- scale quadrangle in southcentral Alaska. (B) Location of part of Healy B-6 15-minute quadrangle. (C) Location of sites 1 and 2 near Toklat River.

1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403. Email for Norm Savage: [email protected] 2Department of Zoology, Oregon State University, Corvallis, Oregon 97331. 3Denali National Park, P.O. Box 9, Denali Park, Alaska 99755.

85 86 SHORT NOTES ON ALASKA GEOLOGY 1999 SAVAGE AND OTHERS

(1990), and an early Frasnian age. Brachiopods include The Mystic subterrane was originally defined by Hypothyridina sp., Variatrypa (Radiatrypa) sp., Jones and others (1981) as a separate tectono- Spinatrypa (Exatrypa) sp., and Eleutherokomma sp. stratigraphic entity of full terrane rank. More recently, Rugose corals are also common, and include both Decker and others (1994) recognized that the Mystic, as colonial forms (notably phillipsastreids and well as the Nixon Fork and Dillinger terranes, were undetermined fasciculate types), as well as solitary genetically related and all were reduced in rank to rugosans. Conodonts from locality 2 were recovered subterranes of a larger terrane, termed the Farewell from nodular, argillaceous limestone beds in the terrane. Gilbert and Bundtzen (1984) considered the uppermost part of the underlying thick unnamed shale Dillinger and Mystic terranes, each of whose type unit. These limestone beds represent a thin transitional sections were close to one another, to represent a single interval with the overlying unit. Conodonts from these stratigraphic succession of Paleozoic to Triassic age, beds include those listed above from the upper unit. preferring to apply the term “sequence” to each. They Brachiopods from these same beds include Schizophoria considered the underlying Dillinger sequence to be a sp. Eleutherokomma sp. and Ladogioides pax. Cambrian to Lower Devonian deep-water succession The massive limestone unit and underlying shale unit that is followed depositionally by the Mystic sequence, are widespread in the area and share strong lithologic which consists of laterally variable Devonian to Triassic similarities with similar Frasnian sections recognized (?) shallow-water to nonmarine sedimentary rocks and near the base of the Mystic sequence of west-central intrusive and extrusive mafic and ultramafic rocks. The Alaska, notably in the Shellabarger Pass area, Talkeetna close stratigraphic relationship between the Dillinger and C-6 Quadrangle, and in the Lime Hills D-4 Quadrangle Mystic sequences (or subterranes) was supported by (Blodgett and Gilbert, 1992). These close lithologic and stratigraphic studies by Blodgett and Gilbert (1992) to faunal ties suggest that the rocks described here form an the southwest in the Lime Hills quadrangle. eastward extension of the Farewell terrane of Decker and others (1994). The areal extent of the Farewell terrane is SYSTEMATIC PALEONTOLOGY shown in Blodgett (1998, fig. 2), showing the component subterranes of the Farewell terrane as well as the location Genus ANCYRODELLA Ulrich and Bassler, 1926 of Shellabarger Pass. ANCYRODELLA PRISTINA Khalymbadzha and The Devonian fossils reported here are of Chernysheva, 1970 significance in providing badly needed timelines within Figure 2.7 the weakly metamorphosed sediments exposed along the Discussion. The single specimen recovered has an north side of the central Alaska Range within Denali asymmetrical platform outline and few but large National Park. Very few Paleozoic age fossil localities nodes. were known in this area previously. It is of further Material. 1 Pa element from locality 1. interest in terms of biotic diversity, since all Devonian collections from this part of the Park before the 1990s consisted of poorly preserved corals. In addition, the Genus MEHLINA Youngquist, 1945 fossils reported here provide important evidence for MEHLINA sp. more correct determination of the stratigraphic Figure 2.9 framework for this region. The Frasnian age Discussion. These Pa specimens have the lateral profile determination and associated brachiopod fauna provide of Ozarkodina but lack the characteristic expanded evidence for correlation of these beds with the Mystic platform margins and are assigned to Mehlina. sequence (or “subterrane”) exposed further to the Material. 2 Pa elements from locality 2. southwest in the McGrath, Talkeetna, and Lime Hills quadrangles. Prior to this study, rocks from this part of Genus POLYGNATHUS Hinde, 1879 the Park had been assigned to the Dillinger terrane by POLYGNATHUS cf. P. ROBUSTUS Klapper and Jones and others (1981) and later to the Nixon Fork Lane, 1985 terrane by Mullen and Csejtey (1986) and Csejtey and Figures 2.5–2.6 others (1992). On the basis of our recently acquired fossil Discussion. This specimen differs from typical Pa data and regional field studies conducted in 1994, we elements of P. robustus in having a convex inner now think the assignment of Dillinger “terrane” is platform margin and smaller anterior denticles on the incorrect (it is present and underlies the Mystic strata free blade. It bears some resemblance to P. ljaschenkoi reported here), and that the assignment of Nixon Fork Kuzmin, 1995, but has a more convex inner platform terrane affinities is also incorrect (no shallow-water margin than that species also. carbonate platform equivalent strata of Ordovician–early Material. 1 Pa element from locality 1. Middle Devonian age are recognized in the region). LATE DEVONIAN (EARLY RASNIAN) CONODONTS ROM DENALI NATIONAL PARK, ALASKA 87

Figure 2. (1–4), Playfordia aff. P. primitiva (Bischoff and Ziegler, 1957). (1) Upper view of USNM 481705 from locality 1. (2) Upper view of USNM 481706 from locality 2. (3) Upper view of USNM 481707 from locality 2. (4) Lateral view of USNM 481708 from locality 1, all x 70. (5–6) Polygnathus aff. P. robustus Klapper and Lane, 1985. Upper and oblique-lateral views of Pa element USNM 471709 from locality 1, x 70. (7) Ancyrodella pristina Khalymbadzha and Chernysheva, 1970. Upper view of Pa element USNM 471710 from locality 1, x 70. (8) Mesotaxis cf. M. falsiovalis Sandberg, Ziegler and Bultynck, 1989. Upper view of broken Pa element USNM 471711 from locality 2, x 70. (9) Mehlina sp. lateral view of Pa element USNM 471712 from locality 2, x 70. (10– 11) Icriodus symmetricus Branson and Mehl, 1934. Upper views of Pa elements USNM 471713-471714, both from locality 1, x 70. (12–13) Possibly juveniles of Icriodus symmetricus Branson and Mehl, 1934, or mature specimens of I. praealternatus Sandberg, Ziegler, and Dreesen, 1992, both from locality 1, x 70. (14) Belodella sp. USNM 471717 from locality 1, x 70. 88 SHORT NOTES ON ALASKA GEOLOGY 1999 SAVAGE AND OTHERS

Genus ICRIODUS Branson and Mehl, 1938 usually include apparatus elements that bear ICRIODUS SYMMETRICUS Branson and Mehl, 1934 numerous fine denticles but our small collections do Figures 2.10–2.13 not include any of these elements. Discussion. These Denali specimens are more slender Material. 1 element from locality 1. than characteristic specimens of I. subterminus Youngquist, 1945, and also differ in having a less ACKNOWLEDGMENTS expanded posterior platform. They are unlike typical specimens of I. symmetricus in having lateral We thank Barbara Savage for processing the samples denticles that almost alternate laterally with the and picking the conodont residues. We also are grateful median denticles, although Dr. Charles Sandberg to Norman Silberling and Wyatt G. Gilbert for bringing (written commun., April 1999), believes that the the 1980 discovery to our attention. Thanks are also specimens in Figs 2.10–11 are unquestionably I. extended to Paul Atkinson for his able assistance in the symmetricus, and those in Figs.2.12–13 look like field. Charles Sandberg and Michael Orchard reviewed juveniles of I. symmetricus or mature specimens of I. the submitted manuscript and provided valuable advice. praealternatus Sandberg, Ziegler and Dreesen, 1992. Material. 8 Pa elements; 5 from locality 1, and 3 from REERENCES locality 2. Bischoff, Guenther, and Ziegler, Willi, 1957, Die Genus MESOTAXIS Klapper and Philip, 1972 Conodontenchronologie des Mitteldevons und des MESOTAXIS cf. M. FALSIOVALIS Sandberg, Ziegler tiefsten Oberdevons: Wiesbaden, Germany, and Bultynck, 1989 Hessisches Landesamt Bodenforschung Figure 2.8 Abhandlungen, v. 22, 136 p. Discussion. The only specimen is broken but enough is Blodgett, R.B., 1998, Emsian (late Early Devonian) present to suggest the species Mesotaxis falsiovalis. fossils indicate a Siberian origin for the Farewell Material. 1 Pa element from locality 2. terrane, in Clough, J.G. and Larson, Frank, eds., Short Notes on Alaskan Geology 1997: Alaska Genus PLAYFORDIA Glenister and Klapper, 1966 Division of Geological & Geophysical Surveys PLAYFORDIA aff. P. PRIMITIVA (Bischoff and Professional Report 118, p. 53–61. Ziegler, 1957) Blodgett, R.B., and Gilbert, W.G., 1992, Upper Figures 2.1–2.4 Devonian shallow-marine siliciclastic strata and Discussion. The specimens of Playfordia aff P. primitiva associated fauna and flora, Lime Hills D-4 differ from typical members of the species in having Quadrangle, southwest Alaska, in Bradley, D.C., and less projecting denticles at the posterior end, as Dusel-Bacon, Cynthia, eds., Geologic Studies in pointed out by Dr. Charles Sandberg (written Alaska by the U.S. Geological Survey, 1991: U.S. commun., April 1999). The presence of this species, Geological Survey Bulletin 2041, p. 106–115. even in its somewhat different form, helps Branson, E.B., and Mehl, M.G., 1934, Conodont studies considerably in determining the age of the collections no. 3—conodonts from the Grassy Creek Shale of because it is known elsewhere only from the Late Missouri: University of Missouri Studies, v. 8, no. 3, falsiovalis to transitans Zones of Ziegler and p. 171–259. Sandberg (1990). Dr. Sandberg has commented Branson, E.B., and Mehl, M.G., 1938, The conodont (written commun., April 1999) that it has never been genus Icriodus and its stratigraphic distribution: reported from the falsiovalis Zones. However, the Journal of Paleontology, v. 12, no. 2, p. 156–166. interpretation by the senior author of ongoing work Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, in the Timan Basin, Russia, is that Playfordia G.D., 1992, Geology and geochronology of the primitiva occurs there in the Late falsiovalis Zone Healy Quadrangle, south-central Alaska: U.S. about 2 m (6.5 ft) below the first occurrence of Palmatolepis transitans. Geological Survey Miscellaneous Investigation Material. 23 Pa elements; 9 from locality 1, and 14 from Series Map I-1961, 63 p., 2 sheets, scale 1:250,000. locality 2. Csejtey, Bela, Jr., Wrucke, C.T., Ford, A.B., Mullen, M.W., Dutro, J.T., Jr., Harris, A.G., and Brease, P.F., Genus BELODELLA Ethington, 1959 1996, Correlation of rock sequences across the BELODELLA sp. Denali fault in south-central Alaska, in Moore, T.E. Figure 2.14 and Dumoulin, J.A., eds., Geologic studies in Discussion. The single specimen appears to belong to a Alaska by the U.S. Geological Survey, 1994: U.S. Belodella apparatus. Devonian species of Belodella Geological Survey Bulletin 2152, p. 149–156. LATE DEVONIAN (EARLY RASNIAN) CONODONTS ROM DENALI NATIONAL PARK, ALASKA 89

Decker, John, Bergman, S.C., Blodgett, R.B., Box, S.E., Klapper, Gilbert, and Philip, G.M., 1972, Familial Bundtzen, T.K., Clough, J.G., Conrad, W.L., Gilbert, classification of reconstructed Devonian conodont W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., apparatuses, Symposium on Conodont Taxonomy: and Wallace, W.K., 1994, Geology of southwestern Geologica et Palaeontologica, Sonderbuch 1, p. 97– Alaska, in Plafker, George, and Berg, H.C., eds., The 114. Geology of Alaska: Boulder, Colorado, Geological Kuz’min, A.V., 1995, Lower boundary of the Frasnian Society of America, The Geology of North America, stage of the Russian Platform: Stratigrafiya, v. G-1, p. 285–310. Geologicheskaya Korrelyatsiya [in Russian], v. 3, Dumoulin, J.A., Bradley, D.C., and Harris, A.G., 1998, no. 3, p. 111–120. Sedimentology, conodonts, structure, and regional Mullen, M.W., and Csejtey, Bela, Jr., 1986, Recognition correlation of Silurian and Devonian metasedimetary of a Nixon Fork terrane equivalent in the Healy rocks in Denali National Park, Alaska, in Gray, J.E., Quadrangle, in Bartsch-Winkler, Susan, and Reed, and Riehle, J.R., eds., Geologic studies in Alaska by K.M., eds., Geologic studies in Alaska by the U.S. the U.S. Geological Survey, 1996: U.S. Geological Geological Survey during 1985: U.S. Geological Survey Professional Paper 1595, p. 71–98. Survey Circular 978, p. 55–60. Ethington, R.L., 1959, Conodonts of the Ordovician Sandberg, C.A., Ziegler, Willi, and Bultynck, Pierre, Galena Formation: Journal of Paleontology, v. 33, 1989, New standard conodont zones and early no. 2, p. 257–292. Ancyrodella phylogeny across Middle–Upper Gilbert, W.G., and Bundtzen, T.K., 1984, Stratigraphic Devonian boundary, in Walliser, O.H., and Ziegler, relationship between Dillinger and Mystic terranes, Willi, eds., Contributions to Devonian palaeontology western Alaska Range, Alaska: Geological Society and stratigraphy, Part 1, Chinese-German Collabora- of America Abstracts with Programs, v. 16, no. 5, tion; Part 2, Various Devonian topics: Frankfurt, p. 286. Germany, Courier Forschungsinstitut Senckenberg, Glenister, B.F., and Klapper, Gilbert, 1966, Upper v. 110, p. 195–230. Devonian conodonts from the Canning Basin, Sandberg, C.A., Ziegler, Willi, Dreesen, Roland, and Western Australia: Journal of Paleontology, v. 40, Butler, J.L., 1992, Conodont biochronology, no. 4, p. 777–842. biofacies, taxonomy, and event stratigraphy around Hinde, G.J., 1879, On conodonts from the Chazy and middle Frasnian Lion mudmound (F2h), Frasnes, Cincinnati group of the Cambro–Silurian, and from Belgium: Courier Forschungsinstitut Senckenberg, the Hamilton and Genesee shale of the Devonian, in v. 150, 87 p. Canada and the United States: Quarterly Journal of Stauffer, C.R., 1938, Conodonts of the Olentangy Shale: the Geological Society of London, v. 35, p. 351–369. Journal of Paleontology, v. 12, no. 5, p. 411–443. Jones, D.L., Silberling, N.J., Berg, H.C., and Plafker, Ulrich, E.O., and Bassler, R.S., 1926, A classification of George, 1981, Map showing tectonostratigraphic the toothlike fossils, conodonts, with descriptions of terranes of Alaska, columnar sections, and summary American Devonian and Mississippian species: description of terranes: U.S. Geological Survey Proceedings of the U.S. National Museum, v. 68, Open-File 81-792, 21 p., 2 sheets, scale 1:2,500,000. 63 p. Khalymbadzha, V.G., and Chernysheva, N.G., 1970, Youngquist, W.L., 1945, Upper Devonian conodonts Conodont genus Ancyrodella from Devonian from the Independence Shale(?) of Iowa: Journal of deposits of the Volga-Kamsky area and their Paleontology, v. 19, no. 4, p. 355–367. stratigraphic significance, in Biostratigraphy and ———1947, A new Upper Devonian conodont fauna paleontology of Paleozoic deposits of the eastern from Iowa: Journal of Paleontology, v. 21, no. 2, Russian Platform and western pre-Urals [in Russian]: p. 95–112. Kazan University, 1, p. 81–103. Ziegler, Willi, and Sandberg, C.A., 1990, The Late Klapper, Gilbert, and Lane, H.R., 1985, Upper Devonian Devonian standard conodont zonation: Courier (Frasnian) conodonts of the Polygnathus biofacies, Forschungsinstitut Senckenberg, v. 121, 115 p. N.W.T., Canada: Journal of Paleontology, v. 59, no. 4, p. 904–951.

GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA David Szumigala,1 Stan P. Dodd,2 and Antonio Arribas, Jr.3

SUMMARY

The Donlin Creek property in southwestern Alaska is with moderate to steep, easterly dips). North-trending a large gold system with mineralization extending more structures appear to be normal faults having minor than 6 mi (9.6 km) in a northeast/southwest direction. The displacement (east–west extensional event). Earlier, Queen–Lewis–ACMA area in the southern part of the northwesterly trending thrust faults, occurring along shale property has the largest gold resource identified to date, beds also have minimal displacements but only minor with an estimated measured and indicated resource of gold mineralization. Deformation has been minimal since 5.4 million oz (167.8 tonnes) of gold at a grade of mineralization. 0.088 oz/ton gold (3.0 g/tonne). Total estimated gold resource at Donlin Creek, including the inferred category, INTRODUCTION is 11.5 million oz (357.7 tonnes). Mineralization is open to depth and along strike. Plutonic-hosted gold deposits have become an Gold mineralization parallels a 5-mi-long (8.2-km- important exploration target in Alaska since the discovery long) Late Cretaceous to early Tertiary bimodal and subsequent operation of the five-million-ounce Fort rhyodacitic and mafic dike swarm intruding mid- Knox gold deposit. The Fort Knox deposit near Fairbanks Cretaceous Kuskokwim Group interbedded graywacke remains the best-documented intrusive-hosted gold and shale. Intrusive contacts are highly irregular along deposit in Alaska. Other plutonic-hosted gold deposits in strike and there are both sill- and dike-like components. Alaska vary dramatically from the Fort Knox model in Dike morphologies dominate in the northeastern Lewis fundamental aspects such as ore mineralogy and and Rochelieu areas, whereas igneous sills are dominant alteration styles (McCoy and others, 1997). Nevertheless, in the 400 Area, and the southern Lewis and ACMA most recent exploration for plutonic-hosted gold deposits areas. Gold mineralization is associated with in Alaska has focused on the Yukon–Tanana uplands of disseminated sulfides, sulfide veinlets and quartz– the eastern Interior. The discovery of the Donlin Creek carbonate–sulfide veining in sericite-altered igneous gold deposit in southwestern Alaska emphasizes that rocks and sedimentary rocks. There is a consistent potential for world-class gold deposits in Alaska is not positive correlation between zones of high fault and restricted to the Yukon–Tanana uplands. fracture density, areas of intense sericite alteration, and The Donlin Creek property, in the Kuskokwim high gold grades. Alteration (sericite formation) and Mountains of southwestern Alaska, is approximately felsic dike crystallization (biotite formation) ages by 300 mi (480 km) west of Anchorage and 15 mi (20 km) 40Ar/39Ar dating overlap and are interpreted to indicate north of the village of Crooked Creek on the Kuskokwim that crystallization of the dikes was closely followed by River (fig. 1, inset), the closest navigable waterway. The sericite and carbonate alteration accompanied by pyrite– property is on approximately 42 mi2 (109 km2) of arsenopyrite–gold mineraliza-tion. Stable isotope and privately owned Native land. Calista Corp., a regional fluid inclusion results suggest that fluids responsible for Native corporation, has patent to subsurface rights, and sericite alteration (and at least part of the mineralization) The Kuskokwim Corp., a Native village corporation, has at Donlin Creek formed by mixing of magmatic water and patent to surface rights. The project is controlled meteoric water. Interpretation of sulfur isotopes suggests 100 percent by Placer Dome Inc. under a lease agreement that at least some sulfur is derived from clastic sedimentary signed with Calista Corp. in March 1995. Calista has the rocks. right to earn up to a 15 percent interest in the project upon Gold mineralization is structurally controlled and completion of a positive feasibility study. Locus of refractory (arsenopyrite-hosted). Higher-grade mineral- exploration activity is in the SE¼ T. 23 N., R. 49 W., ization occurs at the juxtaposition of favorable lithology Seward Meridian (62°03’N latitude, 158°10’W (most favorable is rhyodacite, least favorable is shale) longitude). The property has a 5,400-ft-long (1,650-m- and mineralized shear zones/faults (355°–040° trends long) gravel airstrip for access and an 80-person camp on

1Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, Alaska 99709-3645. Email for David J. Szumigala: [email protected] 2Consulting Geologist, 3732 Magrath Road, Bellingham, WA 98225. 3Placer Dome Exploration, Inc., 240 South Rock Blvd., Suite 117, Reno, NV 89502-2345.

91 92 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

Figure 1. Simplified geology of the Donlin Creek property and location of gold prospects. Topographic base from aerial survey flown by Aeromap U.S. for Placer Dome Exploration, Inc. Grid marks are Universal Transverse Mercator (UTM) projection 1927 North American datum, zone 4. Geology modified from maps by Western Gold Exploration and Mining Co. and Placer Dome Exploration, Inc. Inset map shows location of the Donlin Creek project in southwestern Alaska. GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 93

American Ridge, immediately south of the exploration Donlin Creek, including the inferred category, increased area (fig. 1). to 11.5 million oz (357.7 tonnes) with an average grade The Donlin Creek project is in an area of low of 0.085 oz/ton (2.91 g/tonne) gold at a cutoff grade of topographic relief on the western flank of the Kuskokwim 0.04 oz/ton (1.5 g/tonne) gold (Placer Dome press Mountains. Elevations range from 500 to 1,500 ft (150– release, 2/18/99). 460 m) above sea level. Ridges are well rounded and ridgetops are typically covered with rubble crop and REGIONAL GEOLOGY alpine tundra. Soil solifluction lobes blanket virtually all hillsides; these hillsides are forested with black spruce, The regional geology of southwestern Alaska is tamarack, alder, birch, and larch. Soft muskeg, stunted summarized in Decker and others (1994), Patton and black spruce forest, and discontinuous permafrost are others (1994), and Szumigala (1993, 1996). Metamor- common at lower elevations in poorly drained areas. phosed Early Proterozoic sedimentary and plutonic rocks Placer gold was discovered near the Donlin Creek occur as isolated exposures in southwestern Alaska and property in 1909 and significant lode exploration at serve as depositional basement for Paleozoic units of the Donlin Creek began in the 1980s. WestGold Explora- Ruby, Innoko, and Farewell terranes. The Farewell terrane, tion and Mining Co. identified eight gold prospects over a nearly continuous sequence of Paleozoic continental a 3-mi (5-km) strike length through extensive soil margin rocks over 18,000 ft (5,500 m) thick, underlies sampling (over 10,000 samples), trenching, and drill much of the southwestern Alaska Range and northern programs in 1988 and 1989. These prospects, from north Kuskokwim Mountains and unconformably overlies to south, were named Far Side (formerly called Carolyn), Early Proterozoic units. The predominantly Upper Creta- Dome, Quartz, Snow, Queen, Rochelieu, Upper Lewis, ceous Kuskokwim Group, a post-accretionary basin-fill and Lower Lewis (fig. 1). flysch sequence, is the most extensively exposed unit in Placer Dome began working at Donlin Creek in 1995 the region and is interpreted to have formed one contin- and spent approximately $26 million on the Donlin Creek uous marine embayment that stitched together most of project from 1995 to 1998. Based on WestGold’s work, the terranes of southern and western Alaska by Albian the Lewis–Rochelieu area was deemed the most time. The Kuskokwim Group consists largely of favorable target for a large bulk tonnage gold deposit and interbedded lithofeldspathic sandstone and shale, and in exploration by Placer Dome Exploration Inc. has focused large part rests unconformably on all older rock units. The there. Placer Dome’s exploration efforts have been Kuskokwim Group is at least 7.5 mi (12 km) thick in the largely drill focused, with approximately 39,000 ft region surrounding Donlin Creek and the underlying (11,900 m) of reverse-circulation drilling and 249,280 ft basement rocks are unknown. Late Cretaceous to early (76,000 m) of NQ- and HQ-diameter core drilling from Tertiary plutonic and volcanic rocks intrude and/or 1995 to 1998. Placer Dome also completed 13,850 ft overlie all of the younger units. (4,200 m) of excavator and bulldozer trenching, airborne Two major northeast-trending faults traverse and ground geophysical surveys, and a soil-sampling southwestern Alaska, the Denali–Farewell fault system to program. Placer Dome has discovered several additional the south, and the Iditarod–Nixon Fork fault to the north. prospects on the Donlin Creek property, including Latest Cretaceous and Tertiary right-lateral offsets of Duqum, 400 Area, and ACMA (fig. 1). Placer Dome 56 mi (90 km) to less than 94 mi (150 km) occurred on Exploration Inc. is continuing exploration efforts at both faults (Bundtzen and Gilbert, 1983; Miller and present. Bundtzen, 1988). Numerous high-angle faults are parallel Extensive core drilling by Placer Dome Exploration and conjugate to these large faults. Pre-Tertiary rocks Inc. from 1995 through 1998 defined a large gold have undergone at least two folding phases: open to resource extending from the Queen prospect through the isoclinal folds with 1–2 mile (2–3 km) amplitudes and Lewis (formerly Upper Lewis), Rochelieu, Southwest northeast-trending axes, and later broad folds with 15 mi Lewis (formerly Lower Lewis), and Southeast Lewis (25 km) wavelengths and north-northeast-trending fold (formerly Lower Lewis) prospects to the ACMA area. axes. Regional structural elements have been modeled by The Queen–Lewis area has the largest gold resource right lateral wrench fault tectonics with accompanying identified at the Donlin Creek property, defined by over compressional and tensional stresses (Miller and 175 core holes with drill spacing varying from 165 to Bundtzen, 1988). 650 ft (50 to 200 m) centers. Placer Dome announced an The Kuskokwim Mountains represent one of several estimated measured and indicated resource of 5.4 million latest Cretaceous to earliest Tertiary magmatic belts oz (167.8 tonnes) of gold contained in 51.7 million tons found in southern and western Alaska. The Kuskokwim (57 million tonnes) of gold-bearing material grading Mountains belt consists of calc-alkaline to alkaline 0.088 oz/ton (3 g/tonne) gold, using a 0.06 oz/ton (2 g/ basaltic to rhyolitic volcanic fields, isolated calc-alkaline tonne) gold cutoff. The total estimated gold resource at stocks, felsic to mafic dike swarms, and sub-alkaline to 94 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS alkaline volcano–plutonic complexes (Moll-Stalcup, strike and can have both sill and dike components. Some 1994). Plutonic rocks of the Kuskokwim Mountains sills may be thin apophyses to larger dikes. Sills magmatic belt extend over a northeast-trending area of commonly occur below thick shale horizons within the approximately 540 mi by 120 mi (900 km by 200 km). sedimentary rock package. Regional contact relation- Potassium-Argon (K-Ar) dates from igneous rocks in the ships between sedimentary and igneous rocks are Kuskokwim Mountains belt range from 58 to 77 Ma, typically sharp and generally without metamorphic or whereas K-Ar dates for plutonic rocks range from 61 to metasomatic effects. Chilled margins on igneous bodies 73 Ma, with an average age of 69 Ma (Szumigala, 1996, occasionally occur along all contact types. Individual 1993). Geochemical characteristics of the igneous rocks dikes may be up to 200 ft (60 m) wide, but the average suggest a common arc related petrogenesis for the width is 35 to 70 ft (10–20 m). There is no drill evidence Kuskokwim igneous centers (Szumigala, 1993). Most that these dikes coalesce into a larger plutonic body plutons of the Kuskokwim Mountains magmatic belt within 1,300 ft (400 m) of the surface. have quartz-monzonitic to monzonitic compositions and are calc-alkaline. Petrographic, magnetic susceptibility, IGNEOUS LITHOLOGIES and compositional data for plutonic rocks fit criteria for ilmenite series granitoids and geochemical signatures are Donlin Creek intrusive units comprise a dike swarm; compatible with I-type granitoids. Field relationships and hence, conflicting age relationships are likely. Individual limited laboratory measurements indicate the intrusions dikes and sills pinch and swell throughout the prospect were emplaced at maximum depths of 0.6 to 2.5 mi (1 to areas. Igneous units in the Donlin Creek area have been 4 km). On the basis of previous K-Ar dating, divided into five field categories: aphanitic rhyodacite mineralization is contemporaneous with plutonism at porphyry (RDA), crystalline rhyodacite (RDX), fine- several localities in the Kuskokwim region (Szumigala, grained rhyodacite porphyry (RDF), rhyolite (RHY), and 1993). mafic dikes (MD). Rhyodacite porphyry with aphanitic groundmass and PROPERTY GEOLOGY porphyritic phenocrysts (RDA) and rhyodacite with medium- to coarse-grained crystalline texture (RDX) are Graywacke and shale of the Kuskokwim Group occur the most common igneous units, representing in subequal proportions at Donlin Creek (fig. 2). approximately 80 percent of the dike volume. RDA and Kuskokwim Group rocks generally strike east to RDX can have gradational contacts, probably as textural northwest (280° to 320°) and dip moderately (40° to 60°) differences within one dike, but contacts with distinct to the south. Graywacke varies from a light gray to dark chilled margins also occur. Overall, the rhyodacite units gray color, from fine-grained sandstone to fine-grained have similar mineralogy and characteristics. Textures are conglomerate, is massively bedded to 40 ft (12 m) typical for hypabyssal igneous rocks and vary from thickness and breaks into blocks. Shale and siltstone units porphyritic with very fine-grained matrix (“volcanic”) to have prominent bedding and are good bedding indicators almost coarse-grained equigranular (“plutonic”). Color when present in core. Shale and siltstone units are black, varies from light gray to dark blue-gray and phenocrysts carbonaceous, and occasionally contain fine-grained compose approximately 50 percent of rock volume. (diagenetic?) pyrite. Quartz phenocrysts are subrounded to equant, vary from A northeast-trending, anastomosing, felsic 0.04 to 0.31 in (1 to 8 mm) diameter and represent 10 to (rhyodacite) and mafic (alkali basalt/andesite) dike 20 volume percent. Quartz phenocrysts are typically part- swarm intrudes the Kuskokwim Group sedimentary ially to completely resorbed, embayed, and surrounded rocks at Donlin Creek and crops out over approximately by sericite. Feldspar phenocrysts range from 0.02 to 5 mi (8.2 km) of strike length from American Creek to 0.39 in (0.5 to 10 mm) diameter (average 0.15 to 0.20 in Ophir Creek (figs. 1, 2). In general, igneous units in the [4–5 mm]) and 5 to 40 rock volume percent. There is a Northeast Lewis and Rochelieu areas are dikes with 1:1 to 1:2 ratio between plagioclase and orthoclase. northeast strikes and moderate southeast dips that are Biotite phenocrysts are similar in size to quartz and clearly discordant to bedding. Igneous units in the feldspar phenocrysts and comprise 2 to 5 volume percent. Southeast and Southwest Lewis and ACMA areas are Trace amounts of rutile, sphene, apatite, titanium oxide, mostly sills with northwest strikes and moderate to steep allanite (?) and zircon are present. Red garnet pheno- southwest dips. This morphological change is reflected crysts are present in some core samples, but they are in the bedrock geologic map by the thick mass of extremely rare overall (less than 10 garnets reported in rhyodacite present in the southern Lewis area (figs. 1, 2). 250,000 feet [76.2 km] of drilling). Graphite spherules up In detail, individual rhyodacite body orientations vary greatly. Igneous rocks occur as dikes, sills, and fault- Figure 2 (right). Geologic map of the Queen, Rochelieu bounded bodies. Igneous units are highly irregular along and Lewis prospects, Donlin Creek property. GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 95 porphyry rhyodacite fine rhyodacite mafic intrusions graywacke shale interbedded graywacke & shale shears & faults thrust fault Alaska in Meters Project A’ Contour Elevations Cross-section line Southwest 1000 FEET 200 METERS m Donlin Creek Queen-Lewis Area Geology

T ukki Group Kuskokwim A

275

400 250 Unmapped 375

Unmapped 425 375

400

325

350 350 275 m m T

300

325 m B’ 300 m m Unmapped

275 T

425

250

300 T

400 A’

375 275 350

275

325 250 300 A B

m 275

Unmapped

250 225 175

T 200

150 T 96 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS to 1.1 in (3 cm) in diameter occur locally and indicate a Fine-grained rhyodacite porphyry (RDF) occurs as low magmatic oxidation state. narrow dikes with a fine crystalline matrix and smaller Whole rock analyses of the least altered rhyodacite phenocrysts than other rhyodacite units. Extensive samples available from drill core are shown in table 1 and drilling indicates that RDF occurs throughout the Lewis plot within the rhyolite and rhyodacite fields (fig. 3). and Queen prospects. Maximum apparent thickness in Samples with elevated gold values, high loss-on-ignition core is 70 ft (22 m). RDF contains 5 percent 0.04–0.08 in values and high normative corundum were eliminated (1–2 mm) feldspar phenocrysts and 5 percent 0.04– from this data set. The loss of sodium, potassium, and 0.08 in (1–2 mm) quartz phenocrysts, commonly in a calcium during alteration of feldspar to mica and clay flow-banded-like matrix with wispy hairline graphite minerals produced major oxide analyses that are strongly veinlets. RDF dikes are always strongly altered and no peraluminous. Even the least altered samples of igneous primary minerals for dating have been found. However, rocks from Donlin Creek appear weakly peraluminous RDF dikes fit best in geologic modeling and cross-section due to alteration. It is unclear from the present data building if assumed to be younger than mafic dikes and whether the original magma was also peraluminous. Most older than other rhyodacite units. likely, the Donlin Creek igneous rocks have primary Rhyolite dikes occur at the northern end of the Donlin metaluminous compositions like Late Cretaceous Creek property at the Dome and Duqum prospects. The plutonic rocks throughout the Kuskokwim Mountains rhyolite appears more siliceous than the rhyodacite units (compare Szumigala, 1993). Limited trace-element data and generally is a light gray to cream color. Quartz in table 1 are similar to data from Kuskokwim plutonic phenocrysts have square to slightly rounded shapes that rocks with clear volcanic arc signatures. occupy 20 to 25 percent of the rock volume.

Figure 3. Compositional plot for Donlin Creek dikes (modified from Streckeisen and LeMaitre, 1979). Dike compositional data shown in table 1. GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 97

Mafic dikes exposed at the surface in the southern Sericite from rhyolite dikes at the Dome prospect part of the Donlin Creek property weather a distinct have the youngest 40Ar/39Ar ages (65.1 + 0.9 Ma and 68.0 reddish-brown color with a fine granular texture. Mafic + 1.0 Ma) from the Donlin Creek area. These young ages dikes are generally only 5–10 ft (1.5–3.0 m) thick. suggests that mineralization at Dome may be related to a Sparsely distributed mafic dikes crop out along the different, younger hydrothermal system than that runway on American Ridge, beyond the known southern responsible for gold mineralization at Queen and south limit of mineralization. Field relationships and 40Ar/39Ar Lewis. Alternatively, the Dome rhyolite ages may simply dating indicate that mafic dikes are the earliest igneous reflect a longer time to cool below the argon blocking phase in the Donlin Creek area. Several drill intercepts of temperature for the apparently deeper Dome system. fresh mafic dike are biotite-rich (up to 75 percent) and STRUCTURE hence lamprophyres. Mafic dikes are generally almost completely altered to carbonate and sericite, with a Structural patterns at Donlin Creek are complex and bleached cream to greenish tan color. A characteristic still being deciphered, but several features appear to be alteration/oxidation mineral is bright green fuchsite important in understanding and predicting mineraliza- occurring as isolated grains. Petrographic examination tion. Structural controls appear to be very important in the shows that mafic dikes contain 2–5 percent opaques deposit’s genesis, from ground preparation prior to (including trace amounts of fine-grained gersdorffite emplacement of the dike swarm through possible post- [NiAsS]), 10 percent secondary silica (chalcedony) mineralization displacements. Core logging with core filling voids, up to 80 percent very fine-grained orientation by the clay impression method and deep plagioclase laths (An50), 5 percent possible augite trenching programs begun in 1997 were critical in phenocrysts, and highly variable amounts of biotite deciphering the structural history. phenocrysts (10–75 percent) (John McCormack, written Major faults in the project area are not exposed, but commun.). Three relatively unaltered samples of mafic topographic lineaments and airborne-geophysical data dikes plot in the andesite and quartz andesite fields in interpretation suggest that modern stream channels figure 3. follow fault traces. Miller and Bundtzen (1994) mapped RADIOMETRIC DATING northeast-trending Donlin and Crooked creeks as Cretaceous-age splays of the Iditarod–Nixon Fork fault Results from radiometric dating studies on Donlin and interpreted a right-lateral motion for these faults. Creek igneous rocks and alteration are summarized in Other drainages (American Creek, Dome Creek, and table 2. Age spectra for the 40Ar/39Ar data are shown in Snow Gulch) are interpreted to be fault traces based on figure 4 and separated by rock type. airphoto lineaments and aeromagnetic patterns K-Ar dates for biotite from rhyodacite dikes in the (Szumigala, 1997). Donlin Creek area range from 65.1 + 2.0 to 69.5 + 2.1 Ma Drill core shows numerous shears ranging from (Miller and Bundtzen, 1994). These ages are within the microshears to extensive shear zones. The abundance of age range for igneous rocks (61 to 73 Ma) of the Kusko- shears present in drill core is much greater than indicated kwim Mountains plutonic belt (Szumigala, 1993). by previous mapping and figure 2. North-northeast- and Hydrothermal sericite at Donlin Creek has previously been northwest-trending faults reflect the dominant structural dated at 70.0 + 0.3 Ma by 40Ar/39Ar (Gray and others, trends. Many of the igneous/sedimentary contacts 1997, 1992), and 70.9 + 2.1 Ma by K-Ar (Miller and Bundt- observed in core are structural (shears or faults) rather zen, 1994), indicating that mineralization is broadly than intrusive. contemporaneous with emplacement of the dike swarm. Multiphase rhyodacitic intrusions are both Mafic dikes are the oldest igneous rocks in the Donlin concordant and discordant to sedimentary rock bedding. Creek area. A mafic dike at the Queen prospect yielded Intrusion of these dikes probably occurred during an a 40Ar/39Ar-biotite age of 72.6 + 0.9 Ma and a whole-rock extensional tectonic phase and may have been controlled age of 74.4 + 0.8 Ma. Biotite from a rhyodacite dike at the by anticlinal structures within the Kuskokwim Group Queen prospect yielded a 40Ar/39Ar-plateau age of 70.3 country rocks. Late intrusive phases may have + 0.2 Ma. Sericite 40Ar/39Ar ages from altered feldspar remobilized or dislocated mineralized zones and very phenocrysts in rhyodacite dikes from the Queen–Lewis minor post-mineralization faulting may have offset both area range from 73.6 + 0.6 Ma to 67.8 + 0.3 Ma. Overall, intrusions and mineralization. good plateau ages of igneous biotite and sericitized Most faults and shear zones appear to be sub-parallel feldspar from rhyodacite dikes yield similar results within to rhyodacite bodies and have moderate to steep dips. A analytical error, indicating that alteration (and family of moderately to steeply, east- and west-dipping, mineralization?) ages of igneous rocks are indis- normal or oblique-slip faults that strike between 000° tinguishable from igneous cooling ages. and 030° dissects dikes, sills, and sedimentary rocks 98 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS ORMS CIPW N 1.365 1.304 2.541 2.856 3.119 1.652 1.728 2.823 2.608 0.000 0.000 0.000 N03101 N03537 N04489 211794 211796 208786 217286 N03727 N04467 208787 N03749 G12719 N03101 N03537 N04489 211794 211796 208786 217286 N03727 N04467 208787 N03749 G12719 %% 14.36% 13.59 0.01 0.27 14.25 0.01 14.93 0.21% 0.01% 14.95 0.55% 0.00 0.10 15.55 68.25 0.71 0.28 0.09 70.37 15.25 0.00 1.14 0.18 67.14 14.47 0.10 0.00 0.10 65.41 0.29 0.14 14.20 0.00 66.67 0.48 0.36 13.48 0.15 0.01 68.73 0.27 9.93 0.36 0.16 63.41 0.02 8.91 0.25 0.38 0.20 65.94 0.03 0.57 0.54 0.15 66.03 0.15 0.36 0.37 53.19 0.13 0.13 0.53 47.75 0.36 0.22 47.27 0.90 0.26 0.59 0.32 0.54 Whole-rock analyses of representative, least altered igneous rock types at Donlin Creek igneous rock least altered analyses of representative, Whole-rock 3 3 3 2 5 2 O % 3.05 2.94 2.06 2.38 2.26 3.37 2.65 2.17 2.42 2.11 0.43 1.09 O O O 2 O % 4.19 4.65 4.42 3.63 3.75 3.92 3.13 3.92 3.83 1.44 1.83 1.51 2 2 2 O 2 2 MgOMnONa % % 0.62 0.04 0.38 0.02 0.88 0.09 0.93 0.08 0.85 0.07 0.89 0.03 1.10 0.08 0.92 0.07 0.89 0.06 6.64 0.10 10.73 0.15 13.23 0.14 QuartzCorundumOrthoclase % %Albite %AnorthiteDiopside 29.250 25.964 % %Hypersthene 32.333 %Magnetite % 28.781 33.175Hematite 10.031 27.061 27.783 %Ilmenite 31.456 5.118 26.055 0.000 7.074 22.900Apatite %SUM 18.540 33.822 % 0.410 3.557 10.703 0.000 23.427 21.498 % 0.000 26.699 12.846 0.319 5.579 0.000 23.627 0.558 % 20.215 26.864 0.000 12.180 19.745 0.848 6.271 0.000 0.243 0.358 100.00 29.083 32.456 11.533 0.000 24.817 1.099 0.218 100.00 4.401 0.000 23.937 0.586 17.141 32.200 0.000 100.00 24.380 0.246 1.747 19.670 6.144 0.730 0.000 11.758 12.378 100.00 0.000 0.346 9.495 0.148 22.057 8.255 8.814 0.000 10.735 0.723 100.00 12.802 0.000 19.920 0.367 0.743 25.727 2.199 6.933 10.447 100.00 0.000 0.736 0.000 23.392 4.307 0.378 100.00 0.419 1.095 17.514 6.570 10.798 0.000 0.000 100.00 0.495 25.137 0.390 0.753 3.947 41.142 100.00 0.372 0.000 0.922 6.355 45.835 0.736 100.00 0.000 10.241 1.150 0.324 100.00 1.907 0.000 100.00 0.900 0.569 1.327 0.000 0.713 1.201 0.868 Table 1. Table Rock TypeSample Al RDX RDX RDX RDX RDX RDX RDASample RDA RDA MD MD MD P CaOCr Fe FeO %K 2.06 % 1.48SiO 2.15TiO 2.16LOI 1.57TOTAL 2.61 2.08 % 2.52 2.50 98.81 3.43 2.49 1.88 98.63 3.14 3.50 98.85 2.43 4.82 99.12 2.41 3.34 5.44 99.63 2.18 2.66 5.03 99.27 5.82 99.66 2.49 1.22 5.67 99.00 5.16 5.98 5.64 98.73 6.46 5.64 98.67 6.24 5.87 99.11 98.51 9.01 13.78 12.96 GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 99 Trace Element Geochemistry Trace 208787 75 9 4.5 36 3.9 359 1.0 0.6 3 0.3 2 125 0.5? 15 80 128 SampleNumber211794 C %211796217286 1.06 ppm Ba208786 0.92 1480 ppm208787 1.04 Ce 1395 -- ppm 1920 --Sample Cs ppm --Number -- 1505 ppm211794 Ni ppm -- Co 62.5211796 875 ppm -- ppm217286 Nb -- -- 35 Cu 3.4 ppm208786 ppm -- -- ppm -- Pr -- -- Dy ppm 5.9 10 4.5 10 -- ppm ppm 10 -- Rb Er -- 40 -- 25.5 10 ppm ppm 16 -- -- Sm 40 ppm Eu -- ppm -- 4.6 128 7.6 -- ppm 134 ppm Sr -- 3.2 Gd 2.5 ppm 104 88 -- -- ppm -- Ta -- Ga 1.9 ppm -- 1.3 ppm 220 5.7 -- -- ppm -- 214 1.2 ppm Tb Hf 5.7 332 ppm -- -- 282 ppm -- -- 3.9 Th Ho -- 2.0 19 ppm ------16 La ppm Tn -- 0.9 -- -- 4 ppm ------Pb U 2 6 -- -- 0.9 ppm ------Lu 0.7 V 0.4 31 ------Nd ------17 Yb 4 30 ------Y <5 -- -- 0.3 -- 25 ------0.3 26 -- Zn -- 2.2 ------16 Zr -- 23 18 -- -- 18 60 -- 22 -- 179 165 -- 153 123 100 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

Table 2. 40Ar/ 39Ar and K-Ar dates from the Donlin Creek area, Alaskaa

Prospect Area Rock Mineral Plateau Comments Sample ID Type Dated Age (Ma)b Queen Mafic Dike Biotite/ none Mixed phases with relic ages. [email protected] (MD) whole rock Coarse fraction. Queen Mafic Dike Biotite 72.6 ± 0.9 Fair plateau. Biotite separate [email protected] (MD) from fine-grained rock matrix. Queen Mafic Dike Whole 74.4 ± 0.8 Good plateau. Biotite/whole rock [email protected] (MD) rock separate from fine-grained matrix. Queen Rhyodacite Sericite 73.6 ± 0.6 Good plateau, but possible DC96-240@71m (RDX) 39Ar recoil loss results in an older age than the “true” age. Queen Rhyodacite Biotite 70.3 ± 0.2 Good plateau. [email protected] (RDA) Southwest Lewis Rhyodacite Sericite 70.5 ± 0.2 Good plateau. DC96-217@132m (RDA) Southwest Lewis Rhyodacite Sericite 70.5 ± 0.3 Good plateau. DC96-217@35m (RDA) Southeast Lewis Rhyodacite Sericite 70.9 ± 0.3 Good plateau [email protected] (RDA) Rochelieu Rhyodacite Sericite 67.8 ± 0.3 Bimodal plateau. [email protected] (RDX) Dome Rhyolite Sericite 65.1 + 0.9 Good plateau. DC96-250@131m (RHY) Dome Rhyolite Sericitec 68.0 ± 1.0c Mini plateau = 68.6 ± 0.8; agrees DC96-253@166m (RHY) with isochron age using only low Ca/K fractions. Far Side Rhyodacite Sericite 68.0 ± 3.1c Isochron ages used due to [email protected] (RDA) downstepping plateau. Lewis Gulch Rhyodacite Sericite 72.3 ± 1.4c Downshifting plateau, isochron [email protected] (RDX) age agrees well with low Ca/K fractions plateau age. Snow Gulch Rhyodacite Sericite 70.0 ± 0.3 40Ar/39Ar method. Gray and USGS1 others (1992). Snow Gulch Rhyodacite Sericite 69.5 ± 1.1 Isochron-disturbed sample. Gray USGS2 and others (1997).

West side of Crooked Creek Rhyodacite Biotite 65.1 ± 2.0 K-Ar age, K2O very low. Miller & USGS3 Bundtzen (1994). East side of Dome Creek Rhyodacite Sericite 70.9 ± 2.1 K-Ar age. Miller & USGS4 Bundtzen (1994).

East side of Dome Creek Rhyodacite Biotite 69.5 ± 2.1 K-Ar age, K2O very low. Miller & USGS5 Bundtzen (1994). aDates via 40Ar/39Ar method and analyzed by UAF Geochronology Lab unless noted otherwise, 1 sigma analytical error on ages. bPlateau Age is Best Interpreted Age. cIsochron Age (Ma) = Interpreted Age for more complex or disturbed samples. GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 101

Figure 4. 40Ar/ 39Ar spectra for seri- cite and biotite from Donlin Creek igneous rocks. Mafic dike data are slightly erratic, but yield fair plateaus. Rhyolite samples from Dome show a downstepping plateau, with a mini plateau. Biotite and sericite from rhyodacite samples yield good plateaus. 102 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS across the Donlin Creek property (O’Dea, 1997). A well- The deformation documented above may have been exposed sequence of Kuskokwim Group sedimentary caused by (1) northward stratigraphic imbrication along rocks is present on American Ridge along the airstrip. south-dipping thrust faults, or (2) rotational tilt block The sedimentary rocks are cut by a number of north- development during north-south extension. A pre- northeast- to northeast-striking faults, with fault surfaces liminary interpretation of this evidence strongly suggests dipping moderately east or west and characterized by that the D1 event was a north-directed thrusting event. well-developed down-dip slickenside lineations. Several The development of layer-parallel fabrics within the of the faults have broad, open, drag folds developed in sedimentary and igneous rocks and the imbrication of their hanging walls (O’Dea, 1997). The absence of strata across south-dipping faults are consistent with cleavage development parallel to the axial planes of thrust fault development (O’Dea, 1997). The intrusive these folds and the open geometry of the folds suggest bodies must have been subjected to at least part of the D1 that the folding and faults are a consequence of deformation because pervasive shear fracturing in extensional faulting (O’Dea, 1997). igneous rocks can be traced into layer-parallel shearing Fault surfaces in the Lewis trenches are filled with in the sedimentary rocks. It is concluded that sills were abundant graphite and/or limonite (oxidation of sulfides). intruded during the waning of the D1 event because sed- Broad zones, up to 7 ft (2 m) wide, of intense micro- imentary rock-igneous rock contacts are not displaced fracturing filled with arsenopyrite occur within highly across bedding-parallel D1 faults (O’Dea, 1997). sericitized dikes. Discontinuous, vuggy quartz–carbonate In summary, structural studies by Placer Dome veins occur near fault zones. Quartz–carbonate veins Exploration Inc. hypothesize two deformation events at developed in fault wallrocks dip more steeply than the Donlin Creek (O’Dea, 1997). The D1 event was com- faults and have approximately horizontal fibers. These pressional with north-directed movement, resulting in features are compatible with east-west extension and imbrication of stratigraphy and layer-parallel fabrics. normal displacement. This north-northeast-trending Low-angle faults dominate this event and shaley units shear set correlates well with zones of high-grade gold localized most of the movement. Local imbrication of mineralization. rhyodacite suggests that the D1 event was syn- to post- The amount of displacement on any individual fault dike emplacement. The D2 event was an east-west is less than several feet based on drillhole and trench extensional event that fractured all rock units and led to mapping. Cumulatively, the normal displacement across later localization of gold in open fractures. Post- the fault system in the Lewis area is estimated to be less mineralization movement on any given fault or shear than 100 ft (30 m) (O’Dea, 1997). plane appears to range from a few inches (centimeters) to There is widespread evidence from across the Donlin less than 10 ft (a few meters). Creek property for a deformational event (D1) that preceded the development of the north-south oriented ALTERATION normal fault system (D2) (O’Dea, 1997). Some impor- tant evidence for an early deformational event include: Alteration styles are fairly simple at Donlin Creek. All • Kuskokwim Group strata are tilted and consistently intrusive units are altered and fresh rocks are uncommon. dip shallowly to moderately to the south-southwest. Sericite is the main alteration product replacing both The tilted bedding is cut by north-northeast-striking phenocrysts and matrix. Sericite-dominant alteration normal (extensional) faults. (sericite + illite + kaolinite + carbonates + pyrite) is • Kuskokwim Group strata are often imbricated pervasive, but varies in intensity. Most sericite is a whitish across shallowly south-dipping faults. color, but some plagioclase phenocrysts are replaced by light green sericite (montmorillonite?), especially in less • Shale and siltstone strata commonly display zones altered intrusive rocks. It is unclear whether there was a of bedding-parallel shear and gouge. supergene alteration event at Donlin Creek. Carbonate • Many igneous units mapped in the trenches display replaces phenocrysts and rhyodacite matrix. a pervasive shear fracturing that is sub-parallel to Pervasive carbonate alteration (mostly dolomitic layer-parallel shearing in the sedimentary rocks. In and ankeritic) is common in all igneous rock units. many instances, the same tectonic fabric can be Quartz–carbonate–sulfide veinlets crosscut clay- and traced through bedding and into rhyodacite. sericite-altered rock, but relative timing between sericite • Consistent overprinting relations indicate that and carbonate alteration has not been determined. bedding-parallel fabric and faults pre-date steeper However, relative carbonate alteration intensity appears oriented northeast-trending fabric and shears. indepen-dent of sericite alteration intensity. These relations have been observed in exposures Silicic alteration is weak to absent at Donlin Creek, from American Ridge to the Queen prospect. and confined to weak replacement of porphyry and GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 103 graywacke matrix material. Areas of strong pervasive malachite, sphalerite, scorodite, molybdenite, silicic alteration are localized, and present only in rare stibiconite?, kermesite?, and hematite. Trace amounts of areas of strong stockwork silica veining. enargite were seen in thin section enclosed in an Altered mafic dikes are bleached to a cream color. arsenopyrite grain, and trace amounts of fine-grained Alteration of mafic dikes is dominated by carbonate, with gersdorffite were found in mafic dike samples (John local almost complete carbonate replacement. Carbonate McCormack, 1996, unpublished petrographic report for fracture fillings are also very common. Bright green Placer Dome). fuchsite (chrome-bearing mica) is a distinctive alteration Pyrite is common and appears to be the earliest mineral confined to mafic dikes. sulfide phase. Pyrite is ubiquitous in the rhyodacite Very fine-grained graphite is pervasive throughout phases and occurs as disseminated grains and as most intrusive rocks as disseminated grains in a volume microfracture fillings. Disseminated pyrite is generally percentage range of 0.5 to 3 percent and represented by absent within the siliciclastic section. Disseminated pyrite up to 12 percent carbon in chemical analyses (unpub- in the sedimentary rocks occurs as fine to coarse grains lished Placer Dome data). The graphite is thought to be (up to 0.2 in [5 mm] across) preferentially concentrated a hydrothermal alteration product, but it is not clear near igneous contacts. Most, if not all, of the pyrite is whether graphite is a separate alteration event or part of believed to be epigenetic in origin. Pyrite is also common the sericite and carbonate alteration events. Graphite is in areas of strong quartz–carbonate veining in both common and occurs in open spaces or high porosity igneous and sedimentary rocks. Relative abundance of areas, often with coarse sericite or as fine, isolated, shred- pyrite is not an indicator of gold grade. like fragments, wisps and grains. Graphite content in Arsenopyrite is the dominant gold-bearing mineral at rhyodacite commonly increases near faults/shears and Donlin Creek. Arsenopyrite is deposited later than pyrite sedimentary contacts. An increase in graphite, along with and commonly replaces pyrite in all cases examined an increase in finely disseminated sulfides, imparts a petrographically. Arsenopyrite occurs as fine to very fine distinct blue-gray color to the typical gray-colored grains disseminated in intrusive rocks and as fracture/vein rhyodacite. Coarse graphite clots up to 1.1 in (3 cm) in fillings. Fine-grained arsenopyrite is difficult to diameter occasionally occur in rhyodacite units. The distinguish from disseminated graphite in hand specimen genesis of the graphite clots is not clear. It is unlikely that and visual estimates may vary widely in accuracy. the clots are melted Kuskokwim Group sedimentary rock Marcasite is found as coatings on fine- and coarse- because rhyolite composition magmas are not hot enough grained arsenopyrite. to melt carbon and quartz (major constituents of the Native arsenic occurs as dark gray, granular massive sedimentary rocks). The graphite clots may be condensed to reniform masses and grains commonly associated with methane (Rainer Newberry, written commun.). Methane stibnite in dolomite veinlets. Stibnite commonly occurs could have formed by decomposition of organic materials as disseminated grains and masses within carbonate veins in the surrounding sedimentary rocks at depth (essentially and occasionally as interlocking needles up to 0.08 by a contact metamorphic phenomenon). The rising methane 1 in (2 mm by 2–3 cm) in size in open spaces within would have cooled, oxidized, and eventually condensed quartz–carbonate veins and on fracture surfaces. Stibnite as graphite. replaces arsenopyrite in at least some cases and stibnite- Wall-rock alteration is not megascopically visible in dominant mineralization is most common in sedimentary the Kuskokwim Group sedimentary rocks even in close rock-hosted veins. proximity to highly altered intrusive rocks. However, Sphalerite occurs as rare, scattered grains associated where intrusive sills terminate along strike there is a thin with stibnite in carbonate veins. Galena, tetrahedrite, and (0.5–3 inch [1.2–8 cm] wide) envelope of black clay or sphalerite are found in sedimentary heavy mineral gouge. No contact metamorphic effects were noted in concentrates (Chryssoulis and others, 1996). Realgar and wallrocks except at the Dome and Duqum prospects. orpiment occur as vein and fracture fillings in late, crosscutting structures (+ quartz) and locally replace MINERALIZATION sericite casts of feldspar phenocrysts. Graphite may be deposited last because it typically fills open spaces and ORE MINERALS fractures, but there is not a clear relationship to other Ore mineralogy at Donlin Creek is dominated by mineralization. simple sulfide assemblages. The most common ore Gold mineralization at Donlin Creek is refractory, minerals are pyrite, arsenopyrite, and stibnite. Other ore with arsenopyrite as the dominant gold-bearing mineral. minerals include marcasite, realgar, orpiment, and Geochemical results indicate the affinity of gold with limonite, with lesser amounts of native arsenic, native arsenic and antimony. Metallurgical tests indicate that gold, cinnabar, covellite, chalcopyrite, galena, pyrrhotite, 95–98 percent of the gold in the Lewis area is contained 104 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS in arsenopyrite. Fine-grained arsenopyrite (<20 mm dikes and faults with a subordinate northwest fabric diameter) contains 5–10 times more gold than coarse- occurs in the Northeast Lewis/Rochelieu area whereas grained arsenopyrite (Chryssoulis and others, 1996). north-northeast-trending structures crosscutting Visible gold is extremely rare at the Donlin Creek northwest-trending dikes, sills, and structures dominate property and has only been found at the Far Side prospect the Southeast and Southwest portions of the Lewis areas in association with thin quartz veins cutting rhyodacite and the ACMA area. porphyry. Gold observed in polished thin sections occurs In the Rochelieu area, three mineralized zones over as 1 to 3 mm blebs with no clear paragenetic relationship a 1,000-ft-wide (300-m-wide) area can be traced for to other minerals (John McCormack, 1996, unpublished 2,000 ft (600 m) in a northeast direction. The zones are petrographic report for Placer Dome). 35 to 70 ft (10–20 m) thick, dip 45°–60° to the southeast and average 0.058 to 0.12 oz/ton (2–4 g/tonne) gold. MINERALIZATION ZONES Mineralized zones are hosted in graywacke, shale, rhyodacite with sheared margins, and a rhyodacite dike. Gold mineralization at Donlin Creek occurs over a The Northeast Lewis area contains four mineralized remarkable 4-mi-long (6.5-km-long) area with a north- zones controlled by normal faults having minor east–southwest trend. Mineralization is open at depth and displacement that can be traced for approximately to the northeast and west. Figure 1 shows 12 prospects, 1,600 ft (500 m) in a northeast direction. The ore zones mostly aligned in a northeast-southwest direction along vary in strike from 355o to 045o and dips range from a rhyodacite dike system. Most prospect names refer to vertical to -50°, both to the east and west. The ore zones the creek immediately north of the ridge on which they are similar in thickness to zones described at Rochelieu, occur. From north to south, the prospect names are Far with the addition of a zone of variable thickness that is Side (formerly known as Carolyn or Wheetie), Dome, controlled by a low-angle fault subparallel to a Duqum, Quartz, Snow, Queen, Lewis (further sub- rhyodacite/sedimentary rock contact. Factors that divided into Northeast, Southeast, Southwest, Vortex, and apparently controlled gold deposition include the Rochelieu areas), 400 Area, and ACMA. The bulk of presence of porphyry dikes or thick graywacke units and recent exploration work has been conducted in the possibly flexures in structure orientations. Queen, Lewis, and ACMA areas. Mineralization at the Queen prospect is associated Gold mineralization at Donlin Creek is lithologically with northeast-trending rhyodacite dikes and north- and structurally controlled. Ore mineralization in igneous northeast-trending faults that dip steeply to the southeast. rocks is controlled by disseminated arsenopyrite, shear/ Igneous lithologies are continuous from Northeast Lewis, fracture coatings, and sulfide (pyrite, arsenopyrite, and/ and the known mineralization at Queen occurs over a or stibnite) ± quartz ± carbonate veinlets. Structurally 1,300 ft (400 m) strike length. controlled mineralization typically occurs in the The south Lewis area has multiple controls on gold Kuskokwim sedimentary rocks proximal to dike and sill mineralization, with northeast-trending shear zones like contacts. Quartz–carbonate–sulfide (pyrite, stibnite, and those present at the north Lewis and Queen areas, as well arsenopyrite) veins are the primary mineralized features, as northwest-trending sills and faults. A cross-section of but gold mineralization also occurs in thin, discontinuous the south Lewis area is shown in figure 6. These multiple veinlets and fracture fillings. Veinlets seldom exceed controls lead to discontinuous, discrete mineralization 0.5 in (1 cm) in diameter and most fracture fills are just zones. Some of the thicker zones of mineralization appear thin sulfide coats on fracture surfaces. Realgar and to be southwest dipping. The thickest and highest-grade orpiment with local high gold values occur as late-stage drill intercepts occur where north-northeast-trending fracture filling in sediments and intrusive rocks structures intersect northwest-trending sills. crosscutting other mineralized structures. Realgar and Several gold soil anomalies (>0.5 parts per million orpiment are locally disseminated in rhyodacite. gold) occur over a 2.5 mi (4 km) strike length to the west Mineralization is open to depth and along strike in the of Lewis ridge in an area without rock exposure. Drilling Lewis/Rochelieu area. Figure 5 is a cross-section through in this area to the west of the Rochelieu area encountered the Northeast Lewis and Rochelieu areas. The ore zones several igneous bodies as well as several drill intercepts are concentrated near rhyodacite bodies, but there is not that assayed between 0.088 and 0.15 oz/ton (3–5 g/tonne) a one-to-one correlation. Ore zones are offset and gold. Drilling proved that dikes occur west of the main discontinuous along portions of their strike length due to Lewis Ridge and that gold mineralization also occurs faults, shearing, and local rock competency differences. within that area. Mineralization occurs in two main structural domains Rhyodacite bodies were exposed during trenching within the Queen to ACMA areas (the main bulk and excavation of road building materials in the mineable resource). A series of north-northeast-trending 400 Area. These bodies are southwest of the Lewis area GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 105 and approximately 2,600 ft (800 m) from previously Gold and arsenic have a fairly strong positive known igneous rocks. These rhyodacite bodies have correlation, reflecting the occurrence of gold as sub- northwest strikes (~300°) and appear to be 30 to 70 ft micron particles in the crystal lattice of arsenopyrite. (10–20 m) thick. Sericite-altered rhyodacite porphyry Mercury and antimony correlate with gold to a lesser locally contains up to 0.23 oz/ton (8 g/tonne) gold. degree. There is a general inverse relationship between Drilling encountered significant gold mineralization gold and sodium. Sodium loss is directly related to an hosted in quartz–pyrite–arsenopyrite–carbonate veinlets increase in sericite alteration, and geochemical data from and local realgar veinlets within altered rhyodacite and the Donlin Creek property indicate that gold values immediate wallrocks. increase with increasing sericite alteration. Based on

Rhyodacite Fine-grained Rhyodacite B (Northwest) Drill Section 5100 NE (Southeast) B’ Mafic Intrusion

Kuskokwim Group 96-230 96-19498-476 (Interbedded 96-32498-478 96-195 400 m 400 m 95-178 Graywacke and Shale) Drill Hole Trace

300 m 96-200 300 m Bedding Trace Fault Trace Thrust Fault Trace

200 m 200 m Gold in Drill Intercepts Au>4ppm Au>3 ppm & <4 ppm

100 m 100 m Au>2 ppm & <3 ppm

Au>1 ppm & <2 ppm 98-465 400 Feet 0m 0m 100 Meters Figure 5. Geologic cross-section through Lewis and Rochelieu prospects. Note the abundant dikes and high-grade gold intercepts associated with faults in the dikes.

A (Southwest) Drill Section 1350 NW (Northeast) A’ Rhyodacite Kuskokwim Group (Interbedded Graywacke 5 7 6 5 4 and Shale) -4 7 9 97-412 -4 8 1 9 9 -4 7 9 -4 8 9 Bedding Trace -1 8 9 5 9 300 m 9 300 m Fault Trace

Drill Hole Trace

2 9 200 m -4 8 8 200 m Gold in Drill Intercepts 9 9 98-496 9 6 -4 -2 8 6 9 4 Au>4ppm 96-260 Au>3 ppm & <4 ppm

0 0 96-241 100 m 5 -5 2 8 100 m Au>2 ppm & <3 ppm 9 -4 7 96-290 9 96-303 Au>1 ppm & <2 ppm 98-489 400 Feet

96-269 0m 0m 100 Meters Figure 6. Geologic cross-section through the Southeast and Southwest Lewis prospects. Note that rhyodacite occurs as both dikes and sills and most high-grade gold intercepts occur within faulted rhyodacite. 106 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS silver:gold ratios and copper:gold ratios, it is inferred that are consistent among sample sets and significant to a progressively deeper expression of a hydrothermal understanding more about the deposit’s origin. Table 3 system is exposed from south (Lewis area) to north summarizes observations made during these fluid (Dome area). inclusion studies. Measured vapor phase homogenization Gold mineralization (especially >0.1 oz/ton [> 3 g/ temperatures for two-phase, liquid-rich, primary fluid tonne] gold) at the Donlin Creek deposit is structurally inclusions in vein quartz range from 150° to 260°C controlled and, in the northeastern Lewis area, localized (Roberts, 1993; table 3). Crushing tests reveal abundant within a post-intrusion, small-displacement, normal fault vapor bubble expansion, indicating a minimum of 0.1 system. Individual faults within this extensional system mole percent CO2 (Roberts, 1993). Porphyry and strike between 000° and 030° and dip predominantly epithermal attributes (vein textures and various phases toward the east, reflecting a syn-mineralization east-west within fluid inclusions) were identified (Roberts, 1993), extensional event. The majority of mineralization is but the fluid inclusions were generally too small for hosted within an irregular swarm of rhyodacitic and microthermometric work. subordinate mafic to intermediate sills and dikes of Jim Reynolds (Fluid Inc., Denver) conducted variable thickness and orientation. Mineralization is preliminary qualitative fluid inclusion work in 1996 for hosted, to a lesser degree, within massive graywacke. Placer Dome Exploration Inc. Secondary fluid inclusions Fault orientations commonly change as faults in igneous quartz phenocrysts are interpreted to broadly propagate through rock types of varying competence and record temperature and fluid characteristics associated composition (Ramsay and Huber, 1987). Faults dip more with hydrothermal fluids responsible for pervasive shallowly in incompetent units and steepen abruptly in sericitic alteration. Pervasive sericitic alteration of more competent units. Dilation zones are created around rhyodacite porphyry at Lewis, Rochelieu and Queen is the steeper segments of the faults. At Donlin Creek, dips interpreted to form from dilute fluids in a hydrothermal of the normal faults may change from relatively shallow system with temperatures probably between 200° and to relatively steep as the fault propagates from shale to 350°C (table 3). Shallow, late quartz ± carbonate ± rhyodacite or graywacke. A dilation zone and possible stibnite ± realgar veinlets, with local associated high gold ore shoot may form as a result of this geometry. values, have an inconsistent fluid inclusion population Resultant high-grade ore shoots may be localized with abundant necking and few reliable primary fluid along these normal faults, where they steepened and inclusions, features common in epithermal quartz dilated as they cut through more competent rock types (Bodnar and others, 1985). No daughter minerals were such as rhyodacite and graywacke. Figure 7 depicts an observed and temperatures of homogenization were idealized fault geometry and mineralization pattern. estimated at below 200°C. However, the inherent variability of a small-displacement fault system combined with the highly irregular geom- etries of a dike swarm will create a large degree of variability in the orientation of ore shoots, and likely relatively low degrees of continuity. A model for structural control of gold mineralization at Donlin Creek is given in figure 7. Competency differences between various rock units are important factors in localization of gold mineralization. The brittle response of igneous rocks and graywacke to stress allows for more fracturing and creation of space for movement of ore fluids. Shale and siltstone tend to act more ductilely under stress and absorb stress by bedding plane movements, restricting the amount of open space for later fluid flow. Shale horizons may have acted more ductilely than the interbedded graywacke and magmatic fluids flowed along shale horizons to produce sills capped by thick shale units. Figure 7. Model for development of mineralization zones 2LUID INCLUSIONS at Donlin Creek. Ore shoots developed in dilation zones along normal faults as the faults steepened in A systematic fluid inclusion study at Donlin Creek rocks such as rhyodacite and graywacke, which are has not been conducted, but observations have been made more competent than the shale units. on three occasions on separate sets of samples. Results GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 107

Table 3. Donlin Creek fluid inclusion data

Prospect Sample FIa sizes FIa Number Estimated Measuredb sample no. description Texture %FIs avg/max origin FI shape of phases Temp (°C) Temp (°C) Interpretation Referencec Far Side Breccia vein at least 5 cm wide in feldspar–quartz porphyry with vein margin of prismatic quartz (1–2 mm wide). Vein matrix is very fine-grained quartz. 1 TR-CT1-20m host wallrock euhedral quartz “eyes” in dike 2 <1/10 S irregular–smooth 2 l+v-rich and 3+multi-solid ? fragments in vein euhedral prismatic quartz 20 <3 P irregular–smooth 2 l-rich <200 147–196 with growth zones long, thin brecciated plumose-textured quartz 25 <3 S irregular–smooth 2 l-rich, rare <200 220 (270 multi-solid hydrothermal fluid from shards in breccia vein 3+multisolid inclusions decrepitate) an early, deep? event breccia-vein matrix euhedral clear quartz <1 <1 P irregular 2 <200 (recrystallized?) opaque fragment in microcrystalline quartz ? ? S? ? ? <160 breccia vein

Snow Breccia vein over 7 cm wide with breccia fragments of stibnite-rich, angular quartz matrix and quartz porphyry. 1 TR-ST-5m grains interlocked with plumose quartz (?after 10 10/25 S smooth 2 l+v-rich and 3+multi-solid ? 216–260 (multi-solid early, saline (minimum anhedral quartz and as amorphous silica or inclusions decrepitate 23 eq. wt percent cockade texture around chalcedony?) at >300) NaCl) fluid porphyry rock fragments broken fragments of anhedral quartz 2 <3 S irregular 2 l-rich ? ? earlier vein event infilling around breccia prismatic, euhedral quartz 2 <3 P irregular–smooth 2 l-rich ~200 153 fragments

Lewis Thin (1–2 mm wide), translucent gray quartz vein cutting gray feldspar–quartz porphyry. 1 TR-LT18-225m resorbed and broken quartz “eyes” in porphyritic 10 6/30 PS/S smooth 2 l+v-rich and rare ? phenocrysts, FI’s along wallrock 3+multi-solid, rare 3-phase fractures CO2-rich inclusions quartz breccia fragments 1 <1 PS? irregular ? <200 in wallrock prismatic quartz in vein <1 <1 P irregular ? <200 FIs along fractures late, subhedral quartz vein infill 2–20 <3 PS/S irregular–smooth 2 l-rich <200

Lewis Highly iron-stained breccia vein over 10 cm wide, with fragments of pale green, feldspar–quartz porphyry and clear to translucent quartz. ?1 TR-LT26-146m resorbed and broken quartz “eyes” in porphyritic 25 3/15 PS/S smooth 2 l+v-rich and 3+multi-solid phenocrysts, FIs along wallrock fractures fragments in vein breccia anhedral, broken quartz crystals 5–15 <3 S irregular–smooth 2 l-rich and 3-phase ? ~220 formed at 1–3 km depth? solid present matrix of breccia vein late, euhedral, comb-textured, <1 <1 P irregular, & irregular 2 l-rich <200 drusy quartz l-v ratios FIs along fractures subhedral quartz vug fill 5 <1 S irregular, & irregular 2 l-rich <200 93–101 l-v ratios

Rochelieu sericite-altered rhyodacite igneous quartz phenocrysts scarce -- S ? l-rich 275–300 dilute fluids in hydrothermal 2 DC96-210B, 156.5 m system

Queen sericite-altered rhyodacite igneous quartz phenocrysts scarce -- S ? l-rich 275–300 dilute fluids in hydrothermal 2 DC96-240, 82.9 m system

Southwest Lewis sericite-altered rhyodacite igneous quartz phenocrysts scarce -- S ? l-rich 275–300 dilute fluids in hydrothermal 2 DC96-260, 148.6 m system 108 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

Prospect Sample FIa sizes FIa Number Estimated Measuredb sample no. description Texture %FIs avg/max origin FI shape of phases Temp (°C) Temp (°C) Interpretation Referencec

Lewis sericite-altered rhyodacite igneous quartz phenocrysts scarce -- S ? l-rich 275–300 dilute fluids in hydrothermal 2 DC96-261, 251 m system

Southeast Lewis DC96-244, 82.9 m & sericite-altered rhyodacite abundant necking-down inconsistent irregular no daughter <200 dilute, heated groundwater2 92.5 m, DC96-248, with quartz + carbonate + of FIs, few reliable primary minerals 148.6 m stibnite + realgar veinlets FIs and high gold grades

Dome sericite-altered rhyolite igneous quartz phenocrysts abundant S l-rich, v-rich, >400–450 formed close to magmatic 2 DC96-250, 131 m with multiple microfractures v-rich w/ hypersaline l fluid source

Dome sericite-altered rhyolite l+v-rich >400–450 formed in zone near (typically 2 DC96-251, 62.5 m with quartz stockworks above) a magmatic source

Dome sericite-altered rhyolite low salinity, vapor-rich 2 350–400 represents early magmatic fluid 3 ?(USGS) with quartz stockworks inclusions with significant CO2+CH4

sericite-altered rhyolite high salinity, l-rich with 1 or 2–3 350–400 represents early magmatic fluid 3 with quartz stockworks 2 daughter minerals, halite melts above 450°–550°C

aFI=fluid inclusion, l=liquid, v=vapor, P=primary, PS=pseudosecondary, S=secondary. bMeasured temperature is measured microthermometric homogenization by vapor-phase disappearance and represents the minimum estimated trapping temperature. c(1) Roberts, 1993, (2) Arribus, written communication, 1996, (3) McCoy and others, 1999. GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 109

Sericitized rhyolite with hydrothermal quartz for analysis) is 300°C on the basis of secondary fluid stockwork mineralization from the Dome prospect has inclusion observations in igneous quartz phenocrysts vastly different fluid inclusion characteristics. Fluid from the same samples. Given the higher temperature inclusions are abundant and present along multiple documented at Dome, water-illite fractionation factors microfractures crosscutting quartz phenocrysts in all for Dome are plotted at 400°C. directions. Fluid inclusions at the Dome prospect consist Fluids responsible for sericite alteration (and at least of a vapor-rich CO2-bearing type co-existing with a high- part of the mineralization) at Donlin Creek likely formed salinity, liquid-rich, halite-bearing type, with liquid– by mixing of magmatic water and meteoric water. The vapor homogenization for both types at 340–380°C mixing trend is evident in figure 8. The largest apparent (McCoy and others, 1999). Similar fluid inclusions are magmatic component of the stable isotope data is found less common throughout the rest of the Donlin Creek in samples from the southern Lewis area as well as from property (preliminary results) and homogenize at 220– Dome. On the basis of the available data, hydrothermal 300°C. These fluid inclusions are interpreted to be fluids at the Snow prospect are interpreted to have a larger similar to fluid inclusions found in the core zone of meteoric water component than the south Lewis areas. porphyry-type deposits. On the basis of observations of Fluids from the Queen and Rochelieu prospects are porphyry-type deposits, these fluid inclusions would intermediate between the other two groups. form very close to magmatic fluid sources where both The data are too limited to draw any more con- high-temperature hypersaline and vapor-rich inclusions clusions. It is unclear whether the Dome and Lewis often coexist due to phase separation of a residual aque- prospects are part of the same hydrothermal system or ous magmatic fluid. separate systems. No fundamental isotopic differences are apparent between the Dome and Lewis prospects and STABLE ISOTOPE STUDIES the calculated hydrothermal fluids and alteration intensities are broadly similar. The geochemical signa- Nine sericite samples from sericite-altered feldspar tures at Donlin Creek are like a classic phyllic zone of a phenocrysts in rhyodacite were collected from across the porphyry-type deposit, with some features at the Dome Donlin Creek property for oxygen and hydrogen isotope prospect similar to the potassic zone (Zaluski and others, analyses (table 4). Figure 8 shows the isotopic 1994). Further sampling at Donlin Creek within the composition of calculated hydrothermal fluids mineralized system(s) and beyond the system(s) might responsible for sericite formation and the relationships help to define gradients within the paleo-hydrothermal between those fluids and magmatic and Alaskan system as well as the boundaries of one or more systems. Cretaceous–Tertiary meteoric waters. The temperature Several oxygen, hydrogen, and sulfur isotope used for calculation of fractionation factors between analyses were obtained from samples of stibnite- and water and illite (the main mineral in the sericite collected pyrite-bearing quartz veins in rhyodacite dikes and

Table 4. Donlin Creek oxygen and hydrogen isotope compositions of sericite samples and calculated isotope ratios of associated water

δδδ18 δδδ δδδ18 δδδ No. Sample Number Location O D OH2O DH2O

1 DC96-251@238m Dome + 8.9 - 126 + 5.4 - 101 2 DC96-250@131m Dome + 4.5 - 144 + 1.0 - 119 3 Snow (surface) Snow + 5.8 - 151 + 2.3 - 126 4 DC96-240@71m Queen + 8.3 - 136 + 4.8 - 111 5 DC96-266@411m North Lewis + 6.6 - 143 + 3.1 - 118 6 [email protected] Rochelieu + 8.4 - 147 + 4.9 - 122 7 [email protected] W. of Rochelieu + 5.8 - 146 + 2.3 - 121 8 [email protected] SE Lewis + 11.3 - 117 + 7.8 - 92 9 DC96-238@195m SW Lewis + 12.5 - 106 + 9.0 - 81 c 10 USGS, Gray and others (1997) Snow Gulch + 24.9, + 24.5 -181 (d) aSamples collected by Antonio Arribas, Jr., and analyzed by Geochron Laboratories (Cambridge, Massachusetts), except for #10. Analytical precision reported as better than + 0.2 per mil (δ18O) and + 5 per mil (δD). b H2O calculated for T = 300°C for all samples, except Dome = 400°C. Fractionation factors for sericite (illite/muscovite-water) from Sheppard and Gilg (1996). cδ18O values for quartz and dickite (d). 110 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

20

0 SMOW (Standard Mean -20 Ocean Water) Primary -40 Magmatic Meteoric Water Line Water -60 Cretaceous-Tertiary Calculated -80 surface waters Donlin Creek 9 in Alaska & Hydrothermal central B.C. 8

D (per mil) Fluids

-100 1 4 5 -120 7 2 6 3 -140 Evolved Meteoric Water Path -160 -20 -15 -10 -5 0 5 10 18

O (per mil)

Figure 8. A δ D versus δ 18O plot depicting stable isotope geochemistry of calculated Donlin Creek hydrothermal fluids. Numbers in circles correspond to samples listed in table 4. The stable isotope geochemistry of the Donlin Creek ore fluids can be formed by the interaction of evolved meteoric water with primary magmatic water. Diagram adapted from Taylor (1979) and primary magmatic water is defined somewhat arbitrarily as the calculated water isotopic composition in equilibrium with “normal” igneous rocks or magmas at temperatures greater than or equal to 700°C (Taylor, 1979). Field for Cretaceous–Tertiary surface waters in Alaska and central British Columbia from Zaluski and others (1994). graywacke and shale wallrock (Gray and others, 1997; δ13C value of -22.8 per mil, similar to a δ13C value of - Goldfarb and others, 1990). The heavy oxygen and light 21.9 per mil, from a felsic dike with graphite spots in the hydrogen isotope values are similar to whole rock oxygen Sleetmute D-4 Quadrangle (Rich Goldfarb, 1997, written isotope results from Kuskokwim Group samples (Gray commun.). These carbon isotope values are consistent and others, 1997). The authors concluded that hydro- with reduced carbon (that is, graphite) δ13C values in thermal fluids were derived from multiple sources and igneous rocks, but overlap with values of biogenic carbon may be partially derived by the dehydration of (Faure, 1986). The isotope results are inconclusive as to surrounding sedimentary rocks. Values of δ18O from whether the carbon is derived from inorganic or organic sulfide-bearing quartz veins at Donlin Creek range from sources. 11 to 25 per mil, and the isotopically heaviest values are associated with later, lower temperature sulfide stages OTHER DONLIN CREEK PROSPECTS (McCoy and others, 1999). The δ34S values for sulfides from all ore stages are -10 to -20 per mil, with late stibnite Intrusion/sedimentary rock contact relationships and values as light as -25 per mil, suggesting that at least some hydrothermal alteration and mineralization are roughly sulfur is derived from clastic sedimentary rocks (McCoy similar for the ACMA, 400 Area, Rochelieu, Lewis, and others, 1999). Queen, Snow, Far Side, and Quartz prospects. The Dome Preliminary carbon isotope data from Donlin Creek and Duqum prospects share similar characteristics that was obtained from a graphite bleb in rhyodacite porphyry are different from the above prospects. exposed in a Southeast Lewis trench. The graphite had a GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 111

2AR SIDE PROSPECT with an average grade of 0.1 oz/ton (3.59 g/tonne gold) (Retherford and others, 1989). Both rhyodacitic dikes and The Carolyn prospect of WestGold was renamed the sills are present and crosscut by north- and northeast- Far Side prospect by Placer Dome Exploration. The Far trending mineralized structures. Dikes have northeast Side prospect is the northernmost prospect on the Donlin trends and southeast dips. Northwest-trending sills Creek property. It is near Quartz Gulch and is off-trend containing anomalous gold values crop out to the north. from other prospects. Bedrock exposures are limited to Mineralization appears to be restricted to narrow high- a placer cut in Quartz Gulch and shallow, slumped grade structures. The Snow prospect contains the only trenches. The only visible gold seen on the Donlin Creek oxidized ore resource found to date on the Donlin Creek property is present in quartz veins exposed in trenches property. and in core at the Far Side prospect. Soil samples collected by WestGold averaged 943 parts per billion 400 AREA PROSPECT gold, with a maximum value of 19.3 parts per million gold. Rhyodacite dikes mapped at the prospect by Mineralized rhyodacite at this prospect was WestGold have northwest trends, unlike the northeast discovered in 1996 at several material pits while trend of dikes seen at most Donlin Creek prospects. excavating material for the Lewis Ridge–American Ridge Drilling by WestGold defined a northeast-trending main road and the Lewis Ridge–Snow Gulch road. The mineralized zone at least 650 ft (200 m) long and up to 400 Area Prospect straddles these two roads constructed 100 ft (30 m) wide, with a calculated gold resource of in mostly wetlands without outcrop (fig. 1). 38,400 oz (1,194 kg) (Retherford and others, 1989). Structurally controlled gold mineralization is found Placer Dome drilled three core holes on the Far Side along low-angle, northwest-striking, moderately prospect during 1996 to test intersecting northwest- southwest-dipping sills and bedding plane shears. A trending rhyodacite dikes, north-northeast-trending complex zone of layer-parallel shearing was intersected structures, and coincident strong gold anomalies in soil in American Creek during drilling. The bedding and samples. Spotty, thin, high-grade gold intercepts were structure relationships are consistent with deformational encountered in rhyodacite. features identified as D1 in the Donlin Creek area. In the WestGold mapped a steeply dipping (approximately 400 Area Prospect, the main mineralized unit is a 84°) rhyodacite dike 80 ft (25 m) thick and trending 310° rhyodacite sill. This sill is proximal to the American azimuth in the immediate lower Far Side area. A 50-ft- Creek fault, and most of the unit is logged as sheared or thick (15-m-thick) rhyodacite dike with a 40° east plunge with abundant mineralized fractures (although it is occurs south of the above dike. Most mineralization and unresolved if shearing is really from this particular fault). the only visible gold occur in the southern dike. Other Mineralized veins strike north-northeast with steep dips dikes in the southwest portion of the Far Side prospect both to the northwest and southeast. Although veins trend appear to trend east-west, but the dikes are thin and north-northeast, the mineralized package appears to discontinuous. Sedimentary rocks in this area strike 110° closely follow the sill, probably due to competency and dip moderately shallowly to the northeast. contrasts in this area of high shearing. The sill is projected Shears noted in trenches and drill logs are semi- to intersect the surface just north of the American Ridge– pervasive throughout the Far Side prospect. Also, left- Lewis road and, appears to branch down-dip into multiple lateral offset of the larger, more northerly rhyodacite dike thin sills to the south, still containing gold values, below and the dike orientation immediately south suggest an the American Creek fault. east-west trend to the tectonic fabric of the area. This Numerous, generally thin (<15 ft [<4.5 m] wide) shearing may have provided a weakened avenue for the rhyodacite units found by drilling are not as well smaller porphyry and mafic dikes to follow and could also constrained as the main 400 Area Prospect sill. Drilling explain the spotty higher gold values. along the American Creek–Snow Gulch road penetrated what appears to be a northwest-trending rhyodacite SNOW PROSPECT dike, with a sill-like appendage. High-grade gold mineralization hosted in graywacke in drillhole Soil sampling by WestGold identified an extensive intercepts is possibly due to a nearby mineralized gold soil anomaly (average of 464 parts per billion gold rhyodacite sill not encountered in drilling. Mafic rocks in soils) along the northwest flanks of Snow Ridge encountered during drilling in the 400 Area Prospect are (fig. 1). Trenching and shallow reverse-circulation mostly thin sills that are generally unmineralized. drilling by WestGold in 1989 identified similar geology However, a 65-ft-thick (20-m-thick), well-mineralized and mineralization to that found at Queen and Lewis to mafic dike with a northwest strike and moderate the southwest. WestGold identified a drill-indicated, northeast dip was encountered in one drillhole. kriged resource of 44,000 contained oz (1,368 kg) of gold 112 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

AMERICAN CREEK MAGNETIC ANOMALY units. The dominant igneous rock is a light gray to cream (ACMA) AREA colored, quartz-eye rhyolite. Secondary biotite (potassic alteration) is present in the rhyolite dike in addition to The American Creek Magnetic Anomaly (ACMA) ubiquitous sericite alteration. Igneous breccias and area occurs on the south slopes of Lewis Ridge (fig. 1). equigranular granodiorite are also present (observed only The ACMA area measures 1 mi by 1,750 ft, or 0.33 mi2 in core). Dikes exposed on the surface and in drill core at (1,700 m by 530 m, or 0.9 km2), with the long axis aligned the Dome prospect do not appear to have enough volume nearly parallel to American Creek. to have hornfelsed the Kuskokwim Group sedimentary Airborne-magnetic data in conjunction with soil rocks. This observation suggests that there may be a more sampling data discovered and delineated mineralization extensive plutonic body at depth. in the ACMA area. Drilling in the area intersected Copper mineralization in the form of disseminated sericite-altered rhyodacite porphyry (RDX) with up to 2 chalcopyrite and quartz–chalcopyrite veinlets occurs in percent disseminated graphite. The dominant strike altered intrusive rock and hornfels and appears direction of bedding in the ACMA area is 125° with dips correlative with elevated silver values. Disseminated to the southwest and northeast. A map based on strike and pyrrhotite is common in both igneous and contact dip direction of bedding on a hole-per-hole basis suggests metamorphosed sedimentary units. Elevated gold values an antiformal structure trending 290° or sub-parallel to appear to be associated with disseminated and vein- American Creek. Sills are the dominant igneous style in controlled arsenopyrite and occur over broad zones ACMA and drilling has encountered two thick (in excess (McCoy and others, 1999). Gold mineralization is of 300 ft [100 m]) south-southwest-dipping sill packages “porphyry style” and hosted in both igneous and formed by multiple sill-within-sill intrusions. Sills metamorphic/sedimentary rock types. Fluid inclusions decrease in thickness to the east-southeast. Three dikes from quartz veins indicate filling temperatures in excess (north-northeast trending), up to 80 ft (25 m) thick, were of 400°C, suggesting a magmatic source for fluids and a also documented within the ACMA area. porphyry-style setting for mineralization. Gold mineralization is associated with north- Much of the available geologic and geochemical data northeast-trending quartz–arsenopyrite–pyrite veinlets, indicate that Dome prospect gold mineralization is part fracture coatings of realgar, quartz–stibnite veinlets, and of the 4-mi-long (6.5-km-long) system extending local disseminated acicular arsenopyrite hosted mainly northward from the Lewis prospect. Based on silver:gold within rhyodacite. Gold mineralization is structurally ratios, it can be inferred that a progressively deeper controlled by north-northeast striking structures with expression of a hydrothermal system is exposed from moderate southeast dips. The thickest ore zones occur south (Lewis areas) to north (Dome). Homogenization where northerly structures crosscut rhyodacite. temperatures from fluid inclusion studies also indicate that the Dome prospect may be the deeper, hotter portion DOME PROSPECT of the Donlin Creek hydrothermal system. Alternatively, it may be indicative of lateral fluid flow with Dome being The Dome prospect is the northeasternmost prospect closer to the hotter part of the hydrothermal system. on the property along the northeast trend of the Donlin Nevertheless, other evidence, such as the presence of Creek dike swarm (fig. 1), on the ridge between Quartz younger rhyolite dikes and a hornfels aureole, suggests and Dome creeks. WestGold conducted soil sampling and that the Dome prospect may be part of a different trenching, but never drilled the Dome prospect. Placer hydrothermal system, or a slightly younger, magmatic + Dome Exploration drilled the Dome prospect in 1996 and hydrothermal event has been superimposed on the 1997. Drill targets were in one of the largest untested gold Lewis–Dome system. soil anomalies on the Donlin Creek property. Soil anomalies average 270 parts per billion gold, with a high DUQUM PROSPECT value of 1,541 parts per billion gold (with coincident arsenic and mercury anomalies). Rock chip samples from The Duqum prospect is the southwestern extension of trenches contained up to 10 parts per million gold. the Dome prospect into Quartz Creek. The geology and The geology of the Dome prospect is distinct from mineralization styles are similar to the Dome prospect. most other Donlin Creek prospect areas. Kuskokwim Auger soil samples from this area contained up to 7 parts Group sedimentary rocks have been hornfelsed to per million gold. Coincident gold, arsenic, and mercury intermixed biotite hornfels and greenish gray calc-silicate soil anomalies and aeromagnetic anomalies in the Duqum hornfels adjacent to porphyritic rhyodacite and rhyolite. area were drill-tested in 1997. Some of the hornfels may be intermixed volcanoclastic GEOLOGY AND GOLD MINERALIZATION AT THE DONLIN CREEK PROSPECTS, SOUTHWESTERN ALASKA 113

DONLIN CREEK DEPOSIT GEOCHRONOLOGY accompanied by pyrite–arsenopyrite–gold AND MODEL mineralization. Mineralization occurred almost contemporaneously with emplacement of the Pervasive sericitic alteration occurs within rhyo- intrusions as a magmatic-hydrothermal system dacite intrusive rocks over a 4.3-mi by 1-mi (7-km by 1.5- concentrated gold within a volatile-rich magma. km) area. This large area of alteration is evidence that an The hydrothermal system led to pervasive sericite unusually large magmatic-hydrothermal system was alteration of the intrusions and deposition of gold– active in the Donlin Creek area. Dike and sill emplace- arsenic mineralization. ment and alteration are contemporaneous within limits of 5. East-west extension and development of north- radiometric dating methods. It is possible that alteration trending normal faults occurred during and after occurred as a magmatic degassing event as the igneous emplacement of the intrusive units. bodies were emplaced near the paleosurface. 6. Mixed magmatic and meteoric fluids remobilized The Donlin Creek dikes represent a bimodal (felsic and deposited gold accompanied by graphite and and mafic) assemblage. This assemblage, along with the arsenopyrite in shears. general orientation of the dikes sub-parallel to north- 7. The latest mineralization event deposited gold + northeast-trending normal faults, strongly suggests an stibnite + realgar + native arsenic + quartz + extensional setting for magma emplacement. The carbonate in open spaces within igneous and morphology of igneous rocks at Donlin Creek as a dike sedimentary rocks. swarm instead of the much more common Kuskokwim 8. Faulting occurred throughout the deposit history, volcanoplutonic complex also suggests a relationship but significant post-mineralization faulting is not between extension and magmatism at Donlin Creek (R.J. recognized in the Lewis area of the Donlin Creek Newberry, written commun., 1999). property. Despite intrinsic mineralization variability, a positive one-to-one correlation has been established between fault ACKNOWLEDGMENTS density, alteration intensity, gold tenor, and intrusive rock thickness. Specifically, the thicker the intrusive body, the The authors are grateful to Placer Dome Exploration greater the fault density. The greater the fault density, the Inc. for permission to publish this paper. The data higher and more consistent the grade, and the more presented in this paper are the result of many people from intense the alteration. It appears that the thickest Placer Dome, Calista Corporation, and The Kuskokwim intrusions are cut by the largest number of faults, resulting Corporation working together on the Donlin Creek in the widest ore zones and most intense alteration zones. project. Thanks are especially extended to Rich Moses, Age dating, crosscutting relationships, and mappable Darren O’Brien, and David Barnett for insightful features present in core and trenches suggest the discussions and careful analysis of the plethora of data following geologic and metallogenic chronology at generated during exploration efforts at Donlin Creek. Donlin Creek: Placer Dome Exploration’s research group provided gold 1. Deposition of Kuskokwim Group sediments in an resource calculations. All of the geologists who logged elongate basin formed by wrench fault tectonics drill core over the years at Donlin Creek are thanked for and lithification of these sediments during the mid- their consistently detailed observations. Roland Bartch Cretaceous. and Mark O’Dea of Steffen, Robertson and Kirsten 2. Lithification of the Kuskokwim Group rocks was provided critical structural observations and interpreta- followed by a major deformation event tions. Thanks also are due to Rainer Newberry and characterized by northward stratigraphic Melanie Werdon for their technical reviews that imbrication along south-dipping thrust faults. significantly improved the final version of this paper. 3. Emplacement of intermediate to mafic dikes and sills (oldest), fine-grained rhyodacitic porphyry RE2ERENCES dikes, and rhyodacitic dikes and sills (youngest) in the Late Cretaceous/Early Tertiary, near the end of Bodnar, R.J., Reynolds, T.J., and Kuehn, C.A., 1985, the major deformation event. Rhyolite dikes in the Fluid inclusion systematics in epithermal systems, in Dome area may be part of this igneous event or a Berger, B.R., and Bethke, P.M., eds., Geology and slightly later, separate igneous event. Dikes likely geochemistry of epithermal systems: Reviews in followed structures subparallel to the regional Economic Geology, v. 2, p. 73–97. grain. Bundtzen, T.K., and Gilbert, W.G., 1983, Outline of 4. Crystallization of rhyodacitic rocks was closely geology and mineral resources of upper Kuskokwim followed by sericite and carbonate alteration region, Alaska, in Proceedings of the 1982 symposium 114 SHORT NOTES ON ALASKA GEOLOGY 1999 SZUMIGALA AND OTHERS

on western Alaska geology and resource potential: Miller, M.L., and Bundtzen, T.K., 1988, Right-lateral Journal of Alaska Geological Society, v. 3, p. 101–117. offset solution for the Iditarod–Nixon Fork fault, Chryssoulis, Stephen, Kafritsa, Gina, and Wong, Clarissa, western Alaska, in Galloway, J.P., and Hamilton, 1996, Deportment of gold in the Donlin Creek ores: T.D., eds., Geologic Studies in Alaska by the U.S. unpublished Advanced Mineral Technology Geological Survey during 1987: U.S. Geological Laboratory (London, ON, Canada) report 96/11, 34 Survey Circular 1016, p. 99–103. p. 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PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA: NEW LIGHT ON SOME OLD PROBLEMS Don M. Triplehorn,1 Jeff Drake,2 Paul W. Layer1,2

ABSTRACT

Single-crystal 40Ar/39Ar ages from the Usibelli Group, Healy, Alaska, illustrate the problems and power of using radiometric ages to date sedimentary sequences. For an ash in the Grubstake Formation, at the top of the Usibelli Group, a mineral age of 6.7 ± 0.1 Ma is significantly younger than previous K–Ar (and new 40Ar/39Ar) ages of glass from the same samples. This new age is consistent with stratigraphic controls on the age of the Grubstake Formation. It also better constrains the first stirrings of uplift in the central Alaska Range and is consistent with other age information on uplift. In addition, our data imply that, in some instances, volcanic glass is not an accurate recorder of the age of eruption and can incorporate excess argon that is difficult to detect. For a tonstein near the top of coal bed #6 in the Middle Miocene Suntrana Formation, sanidine and plagioclase single crystals give ages of ~35 Ma, which are clearly too old based on extensive stratigraphic control. The unit that we dated appears to be an air-fall tuff, and it was our expectation that the age of the minerals would reflect the age of deposition of this stratigraphic marker. At this time, neither the origin of these xenocrystic minerals nor the reasons our ages exceed the expected ages by ~20 million years are known. Finally, the similarity of ages and possible relation between our ash unit and the Sugar Loaf volcanics, which have a K-Ar age of 32–34 Ma, are interesting. As a further complication, it seems likely that Sugar Loaf should be much younger than 30+ Ma; first because it overlies sediments identified as Healy Creek Formation, and second because a volcanic landform probably could not survive that long in this setting. Further work is needed to resolve this enigma.

INTRODUCTION

The exposures of the Usibelli Group from the Alaska Range toward the Gulf of Alaska. Near the end Nenana Coal Field near Healy, Alaska, are well- of Miocene time the Alaska Range began to rise across preserved records of Tertiary deposition. Numerous coal this drainage, damming it and creating one or more lakes units, interspersed in a clastic sedimentary sequence, that ultimately spilled northwestward to form the present have been interpreted to record changes in climate in the Tanana River system. The Nenana Gravel overlies the region (Leopold and Liu, 1994; Wolfe, 1994). Although Usibelli Group and represents an alluvial apron the sequence has abundant fossil control, there are few developed on the north side of the Alaska Range as it rose absolute age determinations from ash layers. Our purpose in Pliocene time (Ager and others, 1994, discuss is to report new radiometric ages for the Grubstake and evidence for Pliocene age of the Nenana Gravel). Suntrana formations of the Usibelli Group. The geology The record for much of the Cenozoic geologic of the Healy area was summarized by Wahrhaftig and history of a large part of interior Alaska is contained in others (1969) to accompany publication of eight 15- these rocks. Unfortunately, that record is based on limited minute geologic quadrangles. Figure 1 shows the data. Age assignments are mainly derived from location and figure 2 the stratigraphic column for the correlations with floral assemblages elsewhere in Alaska Usibelli Group at Suntrana on Healy Creek. or on the West Coast of the conterminous United States; The following description of the geologic setting and these in turn are dated by sparse and sometimes significance of the Suntrana Creek locality (fig. 1) is questionable radiometric ages, usually from distant largely taken from Wahrhaftig (1987). After the last localities (see fig. 3, regional stratigraphic section). Our major fall of sea level at the end of the Cretaceous, much new data add some points of increased certainty but of interior Alaska probably was a broad fluvial plain of mostly raise new questions, suggest new implications, low to moderate relief. After a long interval of subaerial and provide guidance for future research. exposure, large areas of slow subsidence developed in which alluvial, lacustrine, and swamp deposits ANALYTICAL METHODS accumulated. During much of Miocene, and perhaps Oligocene, time rivers flowed southward on an extensive For 40Ar/39Ar analysis, samples were sieved and fluvial plain, across the site of the (then non-existent) washed in deionized water, and biotite, feldspar, and

1Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks AK 99775-5780. 2Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-7320. Email for Paul W. Layer: [email protected]

117 118 SHORT NOTES ON ALASKA GEOLOGY 1999 TRIPLEHORN AND OTHERS

Figure 1. Generalized geologic map of the Healy area, central Alaska, modified from Wahrhaftig and others (1994). Circles show the ash sample locations from the Suntrana and Tatlanika creeks of the Usibelli Group. The town of Healy and the Usibelli Coal Mine are also shown. Jumbo Dome, a Quaternary volcano and a prominent geographic feature of the region, is denoted by the triangle. hornblende crystals were hand-picked from both ash trap and a SAES Zr-Al getter at 400°C. The samples layers. In addition, clean glass shards without visible were then analyzed in a VG-3600 mass spectrometer at phenocrysts were selected from the Grubstake ash. The the Geophysical Institute, University of Alaska samples were wrapped in aluminum foil and arranged in Fairbanks. The argon isotopes measured were corrected one of two levels, labeled top and bottom, within for system blank and mass discrimination, as well as aluminum cans of 1 in (2.5 cm) diameter and 1.8 in calcium, potassium, and chlorine interference reactions (4.5 cm) height. Samples of biotite standard Bern-4B, following procedures outlined in McDougall and with an age of 17.25 Ma (C. Hall, oral commun., 1990), Harrison (1988). Where possible, a plateau age was were used to monitor the neutron flux. The samples were determined from three or more consecutive fractions irradiated for 2 megawatt hours in position 5c of the whose ages are within 2 sigma of each other and total uranium-enriched research reactor of McMaster more than 50 percent of gas release. Figure 4 shows University in Hamilton, Ontario, Canada. selected spectra while table 1 gives a summary of all the Upon their return from the reactor, subsamples (5–100 40Ar/39Ar results, with all ages quoted to the ±1 sigma crystals) of the original samples and separates as well as level and calculated using the constants of Steiger and monitors were loaded into 1/16 inch (2 mm) diameter Jäger (1977). holes in a copper tray that was then loaded in an ultra- high vacuum extraction line. The monitors were fused, VOLCANIC ASH IN THE BASAL and samples either fused or step-heated using a 6-watt GRUBSTAKE ORMATION argon-ion laser using the technique described in York and others (1981) and Layer and others (1987). Argon Wahrhaftig and others (1969, p. D24) described two purification was achieved using a liquid nitrogen cold beds of fine white vitric ash, about 13 ft and 24 ft thick, PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA 119

Figure 2. Type section of the Usibelli Group at Suntrana from Wahrhaftig and others (1994). (1) Coal, showing bone and clay partings; (2) bony coal; (3) bone; (4) claystone and shale; (5) siltstone; (6) sandstone, in part cross bedded; (7) pebbles and conglomerate; and (8) schist (unconformity at top). Stratigraphic locations of ash layers dated in this study are shown by arrows. 120 SHORT NOTES ON ALASKA GEOLOGY 1999 TRIPLEHORN AND OTHERS in the lower part of the Grubstake Formation on the east developed on the shale, and includes a fossil forest bed bank of Tatlanika Creek between the mouths of at the base with leaves, cones, roots, stumps, (Wahrhaftig Roosevelt and Hearst creeks (see their fig. 6 for a and others, 1969, p. D24) and even coalified trunks geologic map and specific location). Leopold and Liu standing in growth position that extend 15–20 ft (4.5– (1994, p. 115) note at least 23 ft (7 m) of Grubstake 6.0 m) into the ash. Jack Wolfe identified the megaflora formation below the ashes, and show it on their measured from this outcrop (Wahrhaftig and others, 1969) and section (p. 120) as a basal conglomerate overlain by sand Leopold and Liu (1994) studied the pollen. They and then shale. The lower ash lies directly on a paleosol consider the Grubstake flora to be Late Miocene in age.

Figure 3. Stratigraphic correlation diagram adapted from Ager and others (1994) showing the floristic stages and the radiometric age constraints established from the Cook Inlet and Pacific Northwest. The Alaska Range formations are from Wahrhaftig and others (1969) and have been correlated to the floristic stages. Ages obtained from the Healy area are also shown. Note that the ages for the Grubstake formation are stratigraphically consistent with the floristic ages, while that from coal bed #6 in the Suntrana formation is not. PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA 121

Wahrhaftig’s sample for radiometric dating to 8.3 ± 0.4 Ma (Wahrhaftig, 1987) using the new decay apparently came from the basal part of the lower ash, constants. They also reported an age of 57.3 ± 2.3 Ma for which includes the forest bed that lies directly on the a biotite concentrate (with 10 percent chlorite), and an paleosol. Our dated sample is from this same layer and age of 54.4 ± 2.2 Ma for a muscovite concentrate (50 per- from essentially the same location. The basal part of the cent muscovite, with plagioclase, sanidine, quartz, and upper ash contained biotite and plagioclase that appeared glass). The mica concentrates are both clearly too old datable, but this was not attempted. based on the age of the flora and disagreement with the age of the glass and may represent incorporation of RADIOMETRIC DATES older, inherited grains in the ash. Wahrhaftig and others (1969) reported a conven- Our ash sample contained four components for tional K-Ar age for glass of 8.1 ± 0.4 Ma, recently revised radiometric analysis: glass, biotite, plagioclase, and

39 39

Figure 4. 40Ar/ 39Ar age spectra of representative samples from coal bed #6 at Suntrana Creek and the Grubstake ash at Tatlanika Creek. See table 1 for interpreted ages. 122 SHORT NOTES ON ALASKA GEOLOGY 1999 TRIPLEHORN AND OTHERS

hornblende. Analyses on four glass subsamples (one heating analyses (fig. 4). The remarkable concordance step-heat, three fusions) yielded an 40Ar/39Ar average between feldspar, biotite, and hornblende ages and the age of 8.3 ± 0.1 Ma, essentially identical to the age presence of excess Ar in some samples may imply that obtained by Wahrhaftig (1987). However biotite, the glass age that we and Wahrhaftig obtained was plagioclase, and hornblende gave consistent and influenced by excess Ar and that the younger age of the significantly younger 40Ar/39Ar ages, with interpreted three minerals more accurately reflects the depositional ages of 6.4 ± 0.6 to 6.7 ± 0.1 Ma (see table 1). We did not age of this ash. Either age is consistent with the see any evidence of minerals with ages older than 50 Ma paleobotanical age cited above (fig. 3). However, we and cannot explain the observation of Wahrhaftig (1987), prefer a weighted mean age of 6.7 ± 0.1 Ma for the ten however we do see evidence of excess Ar in some step- mineral subsamples as the age of the Grubstake ash.

Table 1. Summary of 40Ar/39Ar ages from the Healy areaa

Mineral Integrated age Plateau (p) or Comments or fusion age (Ma) isochron (i) age (Ma)

GRUBSTAKE ASH AT TATLANIKA CREEK Feldspar #1 7.6 ± 0.2 fused Feldspar #2 6.9 ± 0.6 fused Feldspar #3 6.9 ± 0.1 6.5 ± 0.1 (p) 3 fraction plateau, 72% of release. Feldspar #4 7.2 ± 0.4 7.0 ± 0.4 (p) 2 fraction plateau, 63% of release. 40 36 6.6 ± 1.5 (i) Ar/ Ari = 644 ± 859 Biotite #1 6.6 ± 0.1 6.6 ± 0.1 Good plateau includes all fractions. Biotite #2 6.6 ± 0.2 fused Biotite #3 6.8 ± 0.1 fused Biotite #4 7.0 ± 0.1 6.9 ± 0.1 (p) Most of plateau in last fraction 40 36 6.7± 0.1 (i) (6.8 ± 0.1 Ma) Ar/ Ari = 303 ± 4 Hornblende #1 11.1 ± 3.5 fused Hornblende #2 6.4 ± 0.6 fused Glass #1 8.2 ± 0.1 fused Glass #2 8.3 ± 0.1 fused Glass #3 8.4 ± 0.1 8.3 ± 0.1 (p) Good plateau, slight excess?? 40 36 Isochron age = 8.2 ± 0.1 Ar/ Ari=302 ± 6 Glass #4 8.4 ± 0.2 fused

COAL BED #6 AT SUNTRANA CREEK Plagioclase #1 33.6 ± 1.0 Ca/K = 7.6 Plagioclase #2 37.8 ± 0.3 Ca/K = 3.3 Plagioclase #3 32.8 ± 0.5 33.4 ± 0.5 (p) Ca/K = 4.5 U shape – 18 Ma reset Plagioclase #4 38.3 ± 0.6 39.8 ± 0.6 (p) Ca/K = 3.6 ragged – 26 Ma reset Plagioclase #5 33.3 ± 1.7 33.3 ± 1.7 (p) Ca/K = not determined 2 step – loss? Sanidine 34.0 ± 0.2 35.7 ± 0.2 (p) Ca/K = 0.04, 15 Ma reset Hornblende #1 44.1 ± 33.5 25 grains, not precise Hornblende #2 -0.3 ± 0.9 50 grains, no radiogenic Ar Hornblende #3 7.5 ± 1.7 100 grains Hornblende #4 -30.5 ± 226.5 No radiogenic Ar Hornblende #5 -0.4 ± 0.5 No radiogenic Ar Hornblende #6 51.5 ± 10.3 none Step heat, not precise

aAges calculated using the constants of Steiger and Jäger (1977) and using the standard MMhb-1 with an age of 513.9 Ma. Best interpreted ages for each sample are shown in bold. PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA 123

RELATIONSHIP TO OTHER LOCALITIES into dark gray, finer-grained and denser clay at the top. About 30 ft (9 m) above the base of the Grubstake Diatoms are sparsely present in the white, chalky formation at its reference section on Suntrana Creek material, suggesting relatively slow rates of deposition. there is a soft white layer 5–6 ft (1.5–1.8 m) thick. This Tiny burrows filled with chalky material from the is very likely equivalent to one or both of the volcanic overlying unit commonly penetrate the dark upper layer. ashes on Tatlanika Creek. Such a correlation is supported These units are interpreted as seasonal, possibly annual, by the Wahrhaftig and others (1969) report that glass deposits of ash washed into the Grubstake lake from from Suntrana Creek has an index of refraction identical surrounding watersheds. The darker, finer-grained upper to that of glass from Tatlanika Creek. part of each couplet, based on the varve-like appearance, The exact location of the volcano is unknown, but may be a winter layer of the finest suspended material, based on thickness and relative coarseness of the ash it slowly deposited after freeze-up of streams and an ice probably was near the Tatlanika Creek sample locality, cover on the lake. which is 37 mi (60 km) northeast of the Suntrana Creek Wolfe (1994, p. 212) states that the Grubstake leaf locality. Absence of air-fall material at Suntrana suggests assemblage “clearly represents coniferous forest, which that during the volcanic eruptions, winds did not carry is consistent with the radiometric age that indicates a ash southwest from the volcano. Note that this was prior probably latest Homerian or early Clamgulchian age.” to or at the beginning of the Alaska Range uplift; hence This reference is to age of the glass cited by Wahrhaftig wind patterns could have been much different from the and others (1969); it also includes, as shown in figure 3, present, bearing in mind that winds are inherently a radiometric age of about 8 Ma for the Homerian– variable and average patterns may not be significant for Clamgulchian boundary at Kachemak Bay. Based on his short-term events. Note also that prior to uplift of the analysis of foliar physiognomy, Wolfe (1994, p. 211) Alaska Range, drainage was generally to the south or suggests a mean annual temperature of about 3°C for southwest over a broad fluvial plain covering a large part Clamgulchian time. Such a temperature implies strong of the Interior and south to the Cook Inlet area. development of seasonal ice on lakes, which is supported Wahrhaftig and others (1969) interpreted the Grubstake by the varve-like character expressed by the ash-rich Formation at Suntrana as, in part, a lake deposit, possibly layers in the lakebeds at Suntrana Creek. The presence related to first stirrings of Alaska Range uplift that of seasonal ice is also suggested by the common ponded south-flowing streams. If so, streams would have occurrence here of scattered, out-sized sand grains and been flowing into the lake from the north, transporting pebbles up to at least ½ inch (1 cm) in diameter; these are Tatlanika ash from areas somewhere west or north of the most likely dropstones transported by floating ice during volcano. The age of the water-borne ash at Suntrana breakup. T. Ager (oral commun., 1999) points out that Creek should be the same as the air-fall ash at Tatlanika Clamgulchian time is now thought to span perhaps from because erosion and transport would have occurred about 3 Ma to 8 Ma and that over such a long period the immediately after the eruption. Therefore, the ash was climate probably varied considerably and thus the deposited almost simultaneously on a forested land climate estimates of Wolfe (1994) may apply to only part surface and in a lake. This implies a shoreline somewhere of the lower Clamgulchian. This does not negate the between the Suntrana and Tatlanika localities. Our argument here for seasonal ice in this locality at about radiometric age of 6.7 Ma constrains the timing of this 6.7 Ma. “first stirrings” of uplift to be younger than was implied The radiometric age for the volcanic ash in the by the 8.3 Ma age of Wahrhaftig (1987). Our age is Grubstake Formation will provide an approximate age consistent with estimates of the onset of uplift of ~5.4 Ma for insect wings recently found by the senior author a few (Ager and others, 1994) based on the McCallum Creek feet below the base of the ash-rich, chalky layers on age from the eastern Alaska Range, and ~6 Ma based on Suntrana Creek. These sparse fossils have the size and apatite fission track ages from Denali (Fitzgerald and general appearance of mosquito wings—not very others, 1993). impressive, but of some interest because they are The senior author examined the Suntrana Creek apparently the only insect fossils reported for the entire section carefully with the intent of obtaining a Tertiary of interior Alaska. A small collection of the radiometric age. However, the soft material contains wings has been sent to David Grimaldi, American Mu- almost no minerals larger than 62 microns (very fine seum of Natural History, New York, for identification. sand) that appear to be primary volcanic phenocrysts. Our radiometric ages from the Tatlanika locality also Moreover, none of the soft, glass-rich material seems to provide a possible connection to the Canyon Village be of air-fall origin. It occurs in varve-like units about ½ lakebeds locality on the Porcupine River, described by in (1 cm) thick, soft and white at the bottom, grading up Kunk and others (1994). They report a slightly reworked 124 SHORT NOTES ON ALASKA GEOLOGY 1999 TRIPLEHORN AND OTHERS tephra layer ranging from 1/16 to 2 in (0.2–5 cm) thick, Leopold and Liu (1994) reported on a long pollen consisting primarily of slightly devitrified glass mixed sequence from Coal Creek, a south tributary of Healy with silt from the enclosing sediments. 40Ar/39Ar ages for Creek that enters Coal Creek 9 mi (14 km) east from its plagioclase from one locality and biotite from two junction with the Nenana River. It should be noted that localities all agree with one another, within the limits of this is about 6 mi (10 km) from the type locality of the analytical precision. Their best estimate of the age of the Suntrana Formation on Suntrana Creek and that the tephra is the mean age of these results, 6.57 ± 0.02 Ma formation increases in thickness from about 764 ft (late Late Miocene). This is very close to our preferred (233 m) at Suntrana Creek to about 1,312 ft (400 m) at age range of 6.7 ± 0.1 Ma for the Tatlanika Creek ash, Coal Creek (Leopold and Liu, 1994, p. 106). and suggests that they might be the same ash. Further The Suntrana Formation flora is correlated with the evidence is required to support this correlation and a Middle Miocene Seldovian floristic stage of the Cook direct comparison of samples is planned, including Inlet area (Wolfe and Tanai, 1980), based mainly on mineralogy, glass morphology, and microprobe radiometric ages in the Lower 48 (Wolfe, 1981). Turner chemistry of individual components. and others (1980) have one K-Ar age for plagioclase of about 16 Ma from the Seldovian floral stage on the SUNTRANA ORMATION, COAL BED #6 Chuitna River not far from its mouth on the north side of TONSTEIN Cook Inlet.

A kaolinitic clay layer (tonstein) occurs at a number RADIOMETRIC DATES of localities at or near the top of coal bed #6, the uppermost unit of the Suntrana Formation. This is an The significance of the phenocryst-bearing tonstein altered volcanic ash, and contains a variety of primary in coal bed #6 was recognized in 1977, as it is just below phenocrysts, including quartz, plagioclase, and the contact between the Suntrana Formation and the amphibole. At Suntrana Creek it is absent on the east side overlying Lignite Creek Formation and thus is an of the creek and about 1 in (2.5 cm) thick at the top of the important lithostratigraphic boundary. Don Turner, coal on the west side. It is approximately this same University of Alaska, Fairbanks, performed thickness about 1 mi (1.6 km) west along the haul road conventional K-Ar analyses of plagioclase concentrates toward Usibelli Coal Mine on Lignite Creek, but is collected by the senior author at Usibelli Coal Mine south overlain by several inches of coal. In the active coal mine of the mouth of Lignite Creek and on the north side of on and south of Lignite Creek this tonstein has different Lignite Creek about 9.5 mi (15 km) east of the mouth. amounts of coal above it, ranging up to at least 4 ft The former yielded an age of 35.5 ± 1.4 Ma and the latter (1.2 m). In all of these places the coal is overlain by sand 27.0 ± 1.1 Ma, a span covering much of the Oligocene or gravel in erosional contact with the coal; thus variable and clearly much older than the expected Miocene age amounts of the coal have been eroded during deposition based on the Seldovian flora (Wahrhaftig and others, of the overlying stream sediments, down to and including 1969). the tonstein. So far this is the only volcanic ash that has We collected an additional sample from an active been found in the Healy area that contains minerals large face at the Suntrana mine, in the NE¼ sec. 5, T. 11 S., R. enough and in sufficient quantity for radiometric dating. 7 W., just south of Lignite Creek (the locality has since 40 39 There are perhaps several dozen other tonsteins within been reclaimed and is no longer available). An Ar/ Ar coal beds around Healy, but they are mostly very thin analysis of a small sanidine separate yielded an age of (¼ in [0.6 cm] or less) and without volcanic phenocrysts about 32 Ma, again older than the age of the Seldovian in the sand-size range. flora and consistent with the earlier K–Ar ages. Plagioclase plateau ages range from about 40 to 33 Ma, AGE O THE SUNTRANA ORMATION BASED and both the plagioclase and the sanidine show evidence ON PALEOBOTANY of Ar loss. This loss, as seen in the first fractions of gas release (fig. 4), is fairly consistent from sample to sample The flora of the Suntrana Formation is poorly known, and can be explained by single-loss diffusion modeling. in part because relatively few samples have been Loss ages ranging from about 15 to 20 Ma are more examined. Wahrhaftig and others (1969) indicate that consistent with the inferred stratigraphic age of this poor induration limited the collection of entire leaves, sample. necessary for identification. In 1992 Tom Ager, USGS, Hornblende ages and Ca/K ratios were highly Denver, with the senior author, collected leaves from the variable. Multigrain samples with Ca/K ratios greater lower part of the Suntrana Formation about 6 mi (10 km) than 20 have ages of 44 and 51 Ma, while two samples from the mouth of Lignite Creek; these have been with lower Ca/K ratios have ages of 0 Ma and one has an identified by Jack Wolfe (Leopold and Liu, 1994, p. 126). age of 7.5 ± 1.7 Ma. Based on the inconsistency of the PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA 125 data and the low radiogenic content of most samples, we surface after the Tertiary sediments, almost 2,000 ft do not ascribe any geologic significance to the (600 m) thick just 6 mi (10 km) to the north, had been hornblende ages. stripped off of the resistant schist during uplift of the Alaska Range. The small patch of Healy Formation DISCUSSION O THE COAL BED #6 below the volcanics might be a problem in this RADIOMETRIC AGES interpretation, as discussed below. In any case, the implication is that the volcano erupted after the Tertiary As shown in figure 3, radiometric ages from coal-bearing sediments had been eroded, and thus that elsewhere in Alaska provide a reasonably consistent the volcano is younger, possibly much younger, than the framework for the ages of the Tertiary floristic stages of ~8 Ma age (Wahrhaftig and others, 1969) determined for Alaska. White and Ager (1994, p.57) interpret the the top of the coal-bearing sediments. Seldovian stage to range in age from about 20 to 13–14 If the volcano is more than 30 Ma old, serious Ma, that is, Lower and Middle Miocene. Correlations in questions arise. If the volcano erupted before most of the figure 3 are from Ager and others (1994) and show the coal-bearing sediments were deposited, could it have Suntrana Formation as lower Middle Miocene. Leopold been preserved by quick burial under Tertiary sediments and Liu (1994; fig. 5, p.126) compare the Wolfe and and then re-exposed by the present erosional episode? Tanai (1980) interpretation of an upper Middle Miocene This seems unlikely for several reasons, including the age with their interpretation (fig. 13, p.129) of an upper presence of a small patch (erosional remnant or first Lower to lower Middle Miocene age for the Suntrana accumulation?) of underlying coal-bearing sediment, Formation. In any case, our age of greater than 30 Ma is and the lack of rhyolite clasts in the nearby Tertiary substantially older than the expected age of somewhere sediments. between about 11 Ma and 17 Ma. At this time we cannot Wahrhaftig (1970) mapped the sediments below the explain this discrepancy. Either the feldspars in the ash volcanics as “upper” Healy Creek Formation. We have layer are older than the depositional age or, less likely, the no information on the nature of these sediments, and correlation of the Suntrana formation with other Cook assume that this correlation was based on lithology and Inlet Seldovian units is incorrect. If the former is true, this stratigraphic position, that is, in depositional contact with would imply that either the eruption that deposited this the underlying basement. Such basal coal-bearing strata ash entrained a substantial amount of old material and no have been assigned to the Healy Creek Formation over younger material was preserved, or the unit is not an air- a wide area, from the Sushana River on the west to Jarvis fall ash deposit, but rather represents reworking and Creek to the east, a distance of 125 mi (200 km) redeposition of an older unit. However, substantial (Wahrhaftig and others, 1969, p. D7-D8). This implies a reworking of sediments is unlikely in a coal swamp. widespread, low-lying erosion surface that subsided to receive initial Tertiary sediments of about the same THE RELATIONSHIP BETWEEN THE SUGAR lithology over a large area. No time relationship is LOA VOLCANICS AND THE USIBELLI GROUP implied, and if these basal strata span a significant time range, the effect would be to lengthen the temporal range Sugar Loaf Mountain, which outcrops about 5 mi of the Healy Formation. This is in fact suggested by the (8 km) south of the Suntrana exposure, is predominantly occurrence of Healy Formation at Rex Creek and rhyolites interpreted in a cross section by Wahrhaftig California Creek that has a distinctly different (lower (1970) as a viscous plug dome. Albanese (1980) Healy Creek) floral assemblage compared to other provided whole-rock K-Ar ages for Sugar Loaf Moun- (upper Healy Creek) strata in the Nenana Coal Field. See tain rhyolites of about 32–34 Ma, consistent with our Leopold and Liu (1994, p. 106) for a discussion of this feldspar ages from coal bed #6. Although the volcanics issue. They conclude that there must be a major lie predominantly on the Paleozoic Birch Creek Schist, unconformity separating lower Oligocene strata from Wahrhaftig (1970) reports that that there are some strata probably Lower Miocene sediments. This could amount interpreted to be (presumably) “upper” Healy Creek to 14 Ma to 20 Ma. The point here is that the basal Formation (Seldovian) between the rhyolite and the Tertiary sediments in interior Alaska may have been underlying schist. As we observed in the Suntrana deposited over a considerable span of time, and formation, either the stratigraphic ages of the units are assignment to the Healy Creek Formation may be just a incorrect, or the radiometric ages are too old. matter of convenience due to a lack of information. Note that Wahrhaftig’s interpretation (1970, cross- If the volcano is older than 30 Ma, the sediments section B-B’) is that Sugar Loaf Mountain is an eroded under the rhyolite may be part of this “lower” Healy remnant of a volcanic landform. Apparently he thought Creek Formation. If so, the relationship to the formation that it had erupted essentially onto the present land on Healy Creek, only about 5 mi (8 km) to the north, is 126 SHORT NOTES ON ALASKA GEOLOGY 1999 TRIPLEHORN AND OTHERS in question. It appears that the only information on the rhyolites probably are part of the Oligocene “lower” floral assemblage on Healy Creek (Wahrhaftig and Healy Creek Formation. If so, the lowest part of the others, 1969) did not include parts of the formation Healy Creek Formation nearby on Healy Creek is likely below the type locality at Suntrana Creek. Thus it is the same age. However, there is no information on the possible that some “lower” Healy Creek Formation is age of this part of the section, and the presence of present along Healy Creek, but this would require a 14– Oligocene sediments would require a major uncon- 20 Ma unconformity within the formation. If the coal- formity within the Healy Creek Formation which is bearing sediments under the Sugar Loaf volcanics are otherwise considered Miocene. There is no reason to found to have an “upper” Healy Creek flora, this would suspect that the K-Ar ages are incorrect, nor is there any suggest that the radiometric ages for the volcanics are far reason to question identification of these sediments as too old. Healy Creek Formation. Further analytical work and Overall there are inconsistencies involving the ages fieldwork are needed to resolve this paradox. of the volcanics, the underlying Healy Creek Formation, and the Healy Creek Formation on nearby Healy Creek ACKNOWLEDGMENTS that cannot be resolved without further information. This work was supported through funding to the CONCLUSIONS geochronology laboratory from the Geophysical Institute. We wish to think Evan Thoms for collecting the Radiometric dating of stratigraphic sections provides sample at Tatlanika Creek. Gary Stricker and Tom Ager badly needed absolute time constraints and allows for provided thorough and helpful reviews that improved the correlation between geographic regions. Volcanic ash manuscript. layers in sedimentary sequences are obvious targets for radiometric dating. However, as the examples here REERENCES illustrate, dating these sequences is not always straight- forward and what might be applicable for one ash might Ager, T.A., Matthews, J.V., Jr., and Yeend, W.E., 1994, not work for another. Pliocene terrace gravels of the ancestral Yukon River For the Grubstake ash, the previously accepted age near Circle, Alaska: palynology, paleobotany, of 8.3 Ma based on K-Ar dating of glass and confirmed paleoenvironmental reconstruction and regional by 40Ar/39Ar dating of the same glass, is probably in error correlation, in Ager, T.A., White, J.M., and Mat- due to the presence of excess argon in the glass. Minerals thews, J.V., Jr., eds., Tertiary Quaternary boundaries: from this ash give a consistent age of 6.7 Ma, which we Quaternary International, v. 22–23, p. 185–206. prefer as representing the depositional age of this ash and Albanese, M.D., 1980, The geology of three extrusive better defining the lower part of the Grubstake Formation bodies in the central Alaska Range: University of and the first stirrings of uplift in the central Alaska Alaska Fairbanks, M.S. thesis (unpub.), 104 p. Range. Fitzgerald, P.G., Stump, Edmund and Redfield, T.F., For the ash in coal bed #6 of the Middle Miocene 1993, Late Cenozoic uplift of Denali and its relation Suntrana Formation, the radiometric ages from to relative plate motion and fault morphology: plagioclase and sanidine minerals is about 35 Ma. This Science, v. 259, no. 5094, p. 497–499. age is clearly too old based on stratigraphic correlation Kunk, M.J., Rieck, H.J., Fouch, T.D. and Carter, L.D., with Cook Inlet stratigraphy and implies a complicated 1994, 40Ar/39Ar age constraints on Neogene geologic history for the minerals in the ash layer. The age sedimentary beds, upper Ramparts, Half-way Pillar spectra from this ash do not exhibit indications of excess and Canyon Village sites, Porcupine River, east- Ar, but rather have evidence of Middle Miocene Ar loss. central Alaska, in Ager, T.A., White, J.M., and Thus, the minerals in the coal bed #6 ash layer either Matthews, J.V., Jr., eds., Tertiary Quaternary bound- were inherited from older eruptions during a Middle aries: Quaternary International, v. 22–23, p.31–42. Miocene eruption, or the ash is a reworked and Layer, P.W., Hall, C.M. and York, Derek, 1987, The redeposited older ash unit. There are compelling derivation of 40Ar/39Ar age spectra of single grains of arguments against each of these hypotheses. hornblende and biotite by laser step-heating, Geo- Related to the problems associated with the coal bed physical Research Letters, v. 14, no. 7, p. 757–760. #6 ash is the age of the Sugar Loaf rhyolite. The Leopold, E.B. and Liu, Gengwu, 1994, A long pollen similarity of the ages between the feldspars in the coal sequence of Neogene age, Alaska Range, in Ager, bed #6 ash and the Sugar Loaf volcanics suggests a T.A., White, J.M., and Matthews, J.V., Jr., eds., relationship between the two. If the 32–34 Ma K-Ar ages Tertiary Quaternary boundaries: Quaternary Inter- of Albanese (1980) are correct, the sediments under the national, v. 22–23, p. 103–140. PRELIMINARY 40AR/39AR AGES ROM TWO UNITS IN THE USIBELLI GROUP, HEALY, ALASKA 127

McDougall, Ian, and Harrison, T.M., 1988, Geochron- Wahrhaftig, Clyde, Wolfe, J.A., Leopold, E.B., and ology and thermochronology by the 40Ar/39Ar Lanphere, M.A., 1969, The coal-bearing group in the method: New York, Oxford University Press, Mono- Nenana Coal Field, Alaska: U.S. Geological Survey graphs on Geology and Geophysics, v. 9, 212 p. Bulletin 1274-D, p. D1–D30. Steiger, R.H. and Jäger, Emily, 1977, Subcommission on White, J.M. and Ager, T.A., 1994, Palynology, geochronology: Convention on the use of decay paleoclimatology and correlation of Middle Miocene constants in geo- and cosmochronology: Earth and beds from Porcupine River (Locality 90-1), Alaska, Planetary Science Letters, v. 36, no. 3, p. 359–362. in Ager, T.A., White, J.M., and Matthews, J.V., Jr., Triplehorn, D.M., Turner, D.L and Naeser, C.W., 1977, eds.,Tertiary Quaternary boundaries: Quaternary K-Ar and fission-track dating of ash partings in coal International, v. 22–23, p. 43–77. beds from the Kenai Peninsula, Alaska—a revised Wolfe, J.A.,1977, Paleogene Floras from the Gulf of age for the Homerian Stage–Clamgulchian Stage – Alaska Region. U.S. Geological Survey Professional boundary: Geological Society of America Bulletin, Paper 997, 108 p. v. 88, no. 8, p. 1156–1160. ———1981, A chronologic framework for Cenozoic Turner, D.L., Triplehorn, D.M., Naeser, C.W. and Wolfe, megafossil floras of northwestern North America and J.A., 1980, Radiometric dating of ash partings in its relation to marine geochronology, in Armentrout, Alaskan coal beds and upper Tertiary paleobotanical J.M., ed., Pacific Northwest Cenozoic biostratig- stages: Geology, v. 8, no. 2, p. 92–96. raphy: Geological Society of America Special Paper Wahrhaftig, Clyde, 1970, Geologic map of the Healy 184, p. 39–47. D-4 quadrangle, Alaska: U.S. Geological Survey Wolfe, J.A., 1994, An analysis of Neogene climates in Geologic Quadrangle Map GQ-806, scale 1:63,360. Beringia, in Cronin, T.M., Ogasawara, K., and Wolfe, ———1987, The Cenozoic section at Suntrana, Alaska, J.A., eds., Cenozoic climate and paleogeographic in Hill, M.L., ed., Cordilleran Section of the changes in the Pacific region: Palaeogeography, Geological Society of America, Centennial field Paleoclimatology, Palaeoecology, v.108, no. 3–4, guide, vol. 1, p. 445–450. p. 207–216. Wahrhaftig, Clyde, Bartsch-Winkler, Susan, and Wolfe, J.A. and Tanai, T., 1980, The Miocene Seldovia Stricker, G.D., 1994, Coal in Alaska, in Plafker, Point flora from the Kenai Group, Alaska: U.S. George, and Berg, H.C., eds., The Geology of Geological Survey Professional Paper 1105, 52 p. Alaska, The Geology of North America: Boulder, York, Derek, Hall, C.M., Yanase, Yotaro, Hanes, J.A. and Colorado, Geological Society of America, v. G-1, Kenyon, W.J., 1981, 40Ar/39Ar dating of terrestrial p. 937 – 978. minerals with a continuous laser, Geophysical Research Letters, v. 8, no. 11, p. 1136–1138.

SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, COOK INLET–MATANUSKA VALLEY OREARC BASIN, SOUTHERN ALASKA Jeffrey M. Trop1, 2 and Kenneth D. Ridgway1

ABSTRACT

The Paleocene–Eocene Arkose Ridge Formation consists of 1,600+ m of sedimentary and volcanic strata that were deposited along the arcward margin of the Cook Inlet–Matanuska Valley forearc basin, southern Alaska. This study reports new petrologic and sedimentologic data from the Arkose Ridge Formation that are used to interpret the provenance, depositional systems, and paleogeography of the forearc basin during the early Cenozoic. Light mineral provenance studies, conglomerate clast counts, and paleocurrent indicators suggest that detritus was derived from local source terranes exposed north of the Castle Mountain fault

system. Arkose Ridge Formation sandstones are enriched in quartz and feldspar (%Q23F67L10; %Qm21F68Lt11) and can be divided into a lower petrofacies rich in metamorphic detritus (%Lv7Ls0Lm93) and an upper petrofacies rich in volcanic detritus (%Lv70Ls0Lm30). Initially, metamorphic-rich detritus was eroded from the plutonic roots of a remnant early Mesozoic magmatic arc, whereas during the later stages of sedimentation erosion of an active Cenozoic magmatic arc provided detritus rich in volcanic lithic fragments. Sedimentological analysis of the Arkose Ridge Formation reveals a progressive upsection change in sedimentary deposystems from gravelly, alluvial fans to sandy, braided streams to tidally influenced, sinuous streams. A relative rise in sea level during the Late Paleocene to Middle Eocene resulted in northward onlap of tidally influenced deposystems towards the basin margin and increased preservation of fine-grained sediment adjacent to the magmatic arc. Interbedded lava flows indicate relative proximity to active volcanic centers during deposition of the Arkose Ridge Formation.

INTRODUCTION

The general sedimentologic model for forearc basins terranes, and paleogeography of the arcward margin of predicts a gradual evolution from deep-marine, through the forearc basin during the early Cenozoic. In addition shallow-marine, to nonmarine deposystems with to its contributions toward a better general understanding progressive infilling of the basin (Ingersoll, 1979; of forearc basin systems, this study may be useful to Dickinson, 1995). Numerous past studies have examined persons evaluating Cenozoic deposits in the upper Cook the sedimentology and petrofacies of ancient marine Inlet and lower Matanuska Valley for potential oil and depositional systems in forearc basins (for example, gas reservoirs (Oil and Gas Journal, 1998). Ingersoll, 1979; Heller and Dickinson, 1985; Busby- Spera, 1986; Morris and Busby-Spera, 1988; Morris and GEOLOGIC SETTING others, 1989). In contrast, few sedimentological studies have been reported from ancient nonmarine deposits of The Cook Inlet–Matanuska Valley forearc basin, a forearc basins (exceptions: Kuenzi and others, 1979; northeastern continuation of the Aleutian forearc basin, Vessel and Davies, 1981; Fulford and Busby, 1993). The is bounded on the southeast by Mesozoic meta- Cook Inlet–Matanuska Valley forearc basin in southern sedimentary rocks of the Chugach subduction complex Alaska (fig. 1) contains thick sequences of well exposed and on the northwest by late Mesozoic–Cenozoic nonmarine strata and thus provides a unique opportunity igneous rocks of the Alaska Peninsula–Aleutian to better understand nonmarine sedimentation in forearc magmatic arc as well as early Mesozoic igneous, basins. This paper examines the Arkose Ridge metamorphic, and sedimentary rocks of the Talkeetna Formation, a 1,600+-m-thick sequence of Paleocene– magmatic arc (figs. 1, 2; Magoon and others, 1976; Eocene sedimentary and volcanic strata deposited along Csejtey and others, 1978; Winkler, 1992). The the arcward (northern) margin of the Cook Inlet– northeastern part of the forearc basin has been uplifted Matanuska Valley forearc basin. The main goals of this and exposed (Matanuska Valley/Talkeetna Mountains) paper are to describe the sedimentology and petrofacies whereas the southwestern part of the basin (Cook Inlet) of the Arkose Ridge Formation in order to reconstruct the is still subsiding (figs. 1, 2; Fisher and Magoon, 1978). evolution of depositional systems, sediment source The forearc basin strata disconformably overlie early

1Department of Earth and Atmospheric Sciences, 1397 Civil Engineering Building, Purdue University, West Lafayette, IN 47907-1397. Email for Jeffrey Trop: [email protected] 2Present address: Department of Geology, Bucknell University, Lewisburg, PA 17837.

129 130 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY

Mesozoic sedimentary, igneous, and metamorphic rocks others, 1977). Along the arcward margin of the forearc of the Talkeetna magmatic arc, an allochthonous basin are northeast-trending high-angle reverse faults, andesitic island arc assemblage that formed at low including the Castle Mountain fault, which experienced paleolatitudes, was translated northward, and accreted to Eocene–Oligocene dextral strike-slip displacement and southern Alaska during the late Mesozoic and early Neogene dip-slip displacement (figs. 1, 2; Grantz, 1966; Cenozoic (Plafker and Berg, 1994). The forearc basin fill Fuchs, 1980; Little, 1990). consists of 1,000–4,000 m of Cretaceous marine strata The Paleocene–Eocene Arkose Ridge Formation, the and 3,000–7,000 m of mostly nonmarine Tertiary strata focus of this study, has a maximum preserved thickness (fig. 3; Kirschner and Lyon, 1973; Winkler, 1992). of 1,600+ m and consists mostly of conglomerate and Volcanic and plutonic rocks exposed along the northern sandstone with subordinate mudstone, coal, tuff, and margin of the basin record late Mesozoic–Cenozoic arc lava flows (Martin and Katz, 1912; Silberman and magmatism in response to northward- to northwestward- Grantz, 1984). Exposed in a narrow, discontinuous directed subduction of the Farallon, Kula, and Pacific outcrop belt 100 km long and 5–10 km wide, outcrops of oceanic plates (Engebretson and others, 1983; Plafker the Arkose Ridge Formation are confined to the north and Berg, 1994). The Border Ranges fault system, a side of the east–west trending Castle Mountain fault Mesozoic thrust fault (MacKevett and Plafker, 1974) system in the Talkeetna Mountains (fig. 2; Winkler, with localized Cenozoic dextral displacement (Pavlis 1992). The base of the Arkose Ridge Formation and Crouse, 1989), separates the forearc basin from the unconformably overlies pre-Cretaceous igneous and Chugach subduction complex (figs. 1, 2; Plafker and metamorphic rocks of the allochthonous Wrangellia

Figure 1. Generalized map showing major tectonic elements of the southern margin of Alaska. Cook Inlet represents part of an actively subsiding forearc basin located between the modern Aleutian magmatic arc to the north (dotted black line) and the Chugach subduction complex to the south (hatched pattern). Uplifted ancient sedimentary deposits of the forearc basin (open-circled pattern) are exposed in the vicinity of the Castle Mountain fault (CMF). Note that the ancient forearc basin deposits unconformably overlie the allochthonous Wrangellia composite terrane (WCT) and the Late Cretaceous–Eocene magmatic arc. Map adapted from Little (1988). Area of detailed geologic map shown in figure 2 is outlined by rectangle along the eastern part of the Castle Mountain fault. SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, SOUTHERN ALASKA 131 composite terrane. Cretaceous forearc basin deposits Inlet (Winkler, 1992). In the western Talkeetna Moun- were either never deposited locally or were uplifted and tains the top of the formation has been eroded and is not eroded during the latest Cretaceous and/or Early exposed (Winkler, 1992). Paleontologic studies of Paleocene. The Arkose Ridge Formation is overlain by megafossil floras (Martin and Katz, 1912; J. Wolfe in Eocene volcanic rocks and/or Quaternary surficial Silberman and Grantz, 1984) and K-Ar radiometric deposits in the eastern Talkeetna Mountains, and dating of interbedded volcanic rocks (Silberman and Miocene and younger sedimentary rocks in upper Cook Grantz, 1984) define the age of the formation as Late

Figure 2. Generalized geologic map of the Matanuska Valley and western Talkeetna Mountains in the western Anchorage 1x3 degree quadrangle. MS = measured stratigraphic section shown on figure 4. See figure 1 for map location. Geology is from Csejtey and others (1978), Winkler (1992), and this study. 132 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY

Paleocene to Middle Eocene (fig. 3; Winkler, 1992). PROVENANCE Age-equivalent finer-grained sedimentary strata and volcanic ash of the Chickaloon Formation are exposed Compositional data from the Arkose Ridge between the Castle Mountain and Border Ranges faults Formation afford an opportunity to study the petrofacies in the Matanuska Valley and northern Chugach of forearc basin strata deposited adjacent to an active Mountains (fig. 2). The Chickaloon Formation consists magmatic arc. The “magmatic arc” provenance field of of ~1,500 m of Paleocene–Eocene fluvial–lacustrine global sandstone provenance schemes is derived mudstone, sandstone, coal, and minor conglomerate (fig. primarily from marine strata deposited in arc-related 3; Barnes, 1962; Little, 1988). sedimentary basins (Dickinson, 1988; Marsaglia and

Figure 3. Age-event diagram showing stratigraphy of Cook Inlet and Matanuska Valley/Talkeetna Mountains, inferred relative base level changes, and published geochronologic and paleontologic data. SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, SOUTHERN ALASKA 133

Ingersoll, 1992). Notably underrepresented in the global phyllosilicate lithic fragments are the dominant schemes are data from nonmarine strata deposited metamorphic grain types. There is a pronounced proximal to the magmatic arc. We also use our upsection increase in volcanic lithic fragments relative to compositional data to better define the Paleocene– metamorphic lithic fragments. (fig. 5; successions 1, 2 =

Eocene paleogeography of the Cook Inlet–Matanuska %Lv7Ls0Lm93; succession 3 = %Lv70Ls0Lm30). Plutonic Valley forearc region by identifying which source rock fragments are common in all Arkose Ridge terranes were exposed during deposition of the Arkose Formation sandstones and consist predominantly of Ridge Formation. coarse-grained granitic fragments comprising METHODS plagioclase feldspar, monocrystalline quartz, biotite, and muscovite (fig. 6B). Individual plutonic rock fragments Our analysis of the Arkose Ridge Formation is from comprise several mineral grains greater than 0.0625 mm. three study sites located in the western Talkeetna Therefore, our point-count tabulations using the Gazzi- Mountains (see fig. 2 for locations). The most laterally and Dickinson methodology reflect the individual vertically extensive outcrop exposures were observed framework minerals within the rock fragments. For this along the northern flank of Arkose Ridge (fig. 2). A new reason, the lithic populations on table 1 and figure 5 do 1,600+-m-thick measured stratigraphic section from that not include plutonic rock fragments (Ingersoll and others location forms the basis for most of the compositional and 1984). Granitic clast types with minor amounts of sedimentological data presented in this paper (fig. 4). We amphibolite, quartz, and siliceous tuff dominate recognize three major lithologic successions within the conglomerates from the lower part of the Arkose Ridge Arkose Ridge Formation based on distinct upsection Formation (fig. 4). changes in lithofacies assemblages, sedimentary INTERPRETATION O COMPOSITIONAL DATA structures, bed geometries, and bed thicknesses (fig. 4). The sedimentology and depositional systems of each On the basis of modal composition and paleocurrent succession is discussed in a section below. data we interpret the Arkose Ridge Formation as being Thirteen sandstone samples were collected over the derived from local igneous and metamorphic source entire measured section at Arkose Ridge, including each terranes exposed north of the Castle Mountain fault (Jg, of the three lithologic successions (see fig. 4 for sample Jps, Kg, TKg on fig. 2; Csejtey and others, 1978; distribution). Medium to coarse-grained sandstones were Winkler, 1992). This interpretation is in agreement with cut into standard petrographic thin sections, stained for the results of a sandstone petrography study of the potassium and plagioclase feldspar identification, and Arkose Ridge Formation by Winkler (1978). Igneous point-counted using an automated stage. Separate clast types constitute more than 80 percent of the tabulations of grain parameters were kept using both the conglomerate composition (fig. 4) and are petro- Gazzi-Dickinson and “traditional” methods (Ingersoll graphically similar to early Mesozoic plutons exposed and others, 1984). Recalculated data are presented in immediately north of the studied outcrops (Jg on fig. 2). table 1 and figure 5 in terms of the Gazzi-Dickinson Diagnostic clast types include quartz–biotite granite, method so that they can be compared with previous hornblende quartz diorite, hornblende–biotite provenance studies such as Dickinson (1988). Two clast granodiorite, amphibolite, and foliated quartz diorite. counts from conglomerates in the lowermost part of the The early Mesozoic granitic rocks likely contributed a section at Arkose Ridge were also obtained (fig. 4). large part of the quartz, feldspar, and mica framework PETROLOGY grains that are abundant in Arkose Ridge Formation sandstones. On the basis of petrographic characteristics, Arkose Ridge Formation sandstones are moderately mica schist fragments containing biotite, muscovite, and to poorly sorted, contain little matrix, and have angular chlorite were most likely derived from pelitic schist to subrounded framework grains. Sandstones have presently exposed on and around Bald Mountain Ridge average framework grain modes of Q23F67L10 and (fig. 2). Volcanic lithic fragments common in the upper Qm21F68Lt11 and average framework mineral modes of part of the section are interpreted as being derived from Qm23P76K1 (table 1). Sandstones classify as feldspathic Paleocene–Eocene volcanic rocks, mainly mafic lava arenites and arkoses that plot mainly within the flows, along the northern margin of the basin. “continental block–basement uplift” provenance field Paleocene–Eocene extrusive volcanic rocks are exposed (QmFLt and QFL plots on fig. 5). Lithic grain throughout the eastern Talkeetna Mountains (Csejtey populations consist of igneous and metamorphic and others, 1978; Winkler, 1992) but only the plutonic varieties (Lv21Ls0Lm79). Lathwork grains composed of roots of the arc are exposed in the western Talkeetna plagioclase phenocrysts in a dark glassy matrix (fig. 6A) Mountains (TKg on fig. 2). Volcanic lithic fragments dominate the volcanic lithic population. Foliated were eroded, mixed with epiclastic nonvolcanic detritus, 134 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, SOUTHERN ALASKA 135

Figure 4 (left). Detailed measured stratigraphic section of the Arkose Ridge Formation using the lithofacies schemes of Miall (1978). See figure 2 for location of measured section. Stratigraphic range of major lithofacies successions shown by black vertical bars: FS1 = succession 1; FS2 = succession 2; FS3 = succession 3. Grain sizes on horizontal scale: M = mud; F = fine-grained sand; C = coarse-grained sand; P = pebble; B = boulder. Rose diagram at 1,060 m represents paleocurrent data collected from planar cross-stratification in succession 2. Solid circles represent sandstone samples collected for modal analysis. Pie diagrams in succession 1 (0 m and 82 m) represent conglomerate clast compositions. Clast compositional data was obtained by counting all pebble-, cobble-, and boulder-sized clasts within a delineated rectangle on an outcrop face. Note the relative abundance of plutonic clasts (P). Other clast types include tuff (T), quartz (Q), minor metamorphic and sedimentary varieties (O). N = number of clasts counted.

Figure 5 (right). Ternary diagrams showing modal compositions of Arkose Ridge and Chickaloon Formation sandstones. Each symbol represents one modal analysis of 400 or more framework grains using the Gazzi-Dickinson point-count method (Ingersoll and others, 1984) as part of this study or previous studies (Winkler, 1978; Little, 1988). Q = monocrystalline quartz, chert, and polycrystalline quartz; Qm = monocrystalline quartz; Qt = total quartzose grains; P = plagioclase feldspar; K = potassium feldspar; F = plagioclase and potassium feldspar; Lt = total lithic grains; Lv = volcanic lithic fragments; Lm = metamorphic lithic fragments; Ls = sedimentary lithic fragments; L = lithic grains except quartzose lithic types. “Recycled orogen,” “magmatic arc,” “continental block-basement uplift,” and “mixed” provenance fields from Dickinson and others (1983). 136 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY e and K-spar); L=lithic grains (sedimentary, volcanic, and metamorphic lithic e and K-spar); L=lithic grains (sedimentary, ic lithic grains); Qp=polycrystalline quartz; P=plagioclase feldspar; K=potassium 5.80 7.52 3.97 5.87 7.31 3.66 6.87 7.29 0.81 28.24 0.00 28.24 23.19 66.53 10.28 20.64 67.78 11.58 23.41 75.87 0.72 21.22 0.00 78.78 25.38 63.94 10.69 22.9115.90 65.27 75.18 11.81 8.92 26.05 13.05 73.08 76.13 0.88 10.82 6.73 14.61 0.00 85.20 0.19 93.2 69.52 0.00 30.48 22 17.63 70.30 29.10 12.063 56.223 14.683 16.12 15.85 70.79 16.59 71.95 27.48 13.08 15.26 76.59 12.20 57.51 76.99 15.01 6.83 18.55 7.74 12.13 80.91 73.02 0.54 32.34 14.95 14.85 12.06 65.87 76.96 1.80 78.42 21.15 8.09 14.24 9.51 0.00 10.17 85.47 78.85 16.27 0.29 0.00 13.33 83.47 89.8 0.27 86.67 76.00 0.00 0.00 67.86 24.00 64.71 0.00 0.00 32.14 35.29 122 28.172 22.25 62.912 26.06 68.502 8.92 33.57 57.752 9.25 29.02 55.48 16.20 22.98 65.12 10.95 24.22 22.72 60.39 19.70 64.27 5.85 24.76 68.64 16.63 69.19 11.51 31.71 58.57 11.11 8.64 56.83 16.67 24.04 19.85 27.37 11.46 68.29 62.85 22.16 70.73 21.89 29.71 7.67 1.90 17.30 75.57 69.15 35.81 69.14 2.27 1.14 63.36 8.96 26.04 0.83 24.00 8.11 73.96 75.69 5.41 0.00 24.04 0.00 0.31 5.80 0.00 0.00 75.96 91.89 0.00 0.00 94.59 0.00 94.20 4.17 0.00 100.00 0.00 0.00 0.00 100.00 95.83 0.00 100.00 2 22.25 74.08 3.67 19.35 75.29 5.36 20.44 79.56 0.00 12.50 0.00 87.50 Facies . Recalculated point-count data from the Arkose Ridge Formation . Recalculated point-count data from position (m) succession Q F L Qm F Lt Qm P K Lv Ls Lm Stratigraphic Table 1 Table Standard deviation (all data) Grain parameters: Q=quartzose grains (monocrystalline quartz, chert, and polycrystalline quartz); F=feldspar (plagioclas feldspar; Lv=volcanic lithic grains; Lm=metamorphic Ls=sedimentary grains. LONE1-132 LONE1-208 LONE1-326 LONE1-577 LONE1-612 LONE1-766 LONE1-892 LONE1-1071 LONE1-1197 LONE1-1225 (successions 1, 2) Average LONE1-1570 LONE1-1572 LONE1-1575 (succession 3) Average (all data) Average volcanic, metamorph grains); Qm=monocrystalline quartz; Lt=total lithic grains (chert; polycrystalline and sedimentary, SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, SOUTHERN ALASKA 137 and transported southward. The mixing of Cenozoic packages that are 5–15 m thick and laterally continuous volcanic detritus with Mesozoic granitic and at the outcrop scale (10–50 m). Some conglomerate metamorphic detritus produced a more feldspathic and packages fine upwards into 2–5 m thick units of volcanic lithic-rich petrofacies than found in sandstones sandstone or, more rarely, mudstone. Sandstones are from the lower part of the section (fig. 5). Sediment medium to thick bedded, are laterally persistent, and derivation from northern source terranes is consistent commonly exhibit medium-scale, planar cross- with southwestward-directed paleocurrent indicators stratification. Mudstones, which constitute less than (1,060 m on fig. 4) and regional textural data that record 20 percent of succession 1, are generally massive, a southwestward reduction in grain size of Paleocene– carbonaceous, and rich in plant debris. Matrix-rich Eocene strata from the Talkeetna Mountains to the upper conglomerates, although rare, are also present in Cook Inlet (Barnes, 1962; Kirschner and Lyon, 1973; succession 1. The matrix-rich conglomerates are laterally Little, 1988; Winkler, 1992; J.M. Trop, unpublished data). continuous and 10–50 cm thick. Clasts in these Available compositional data from Paleocene– conglomerates are more poorly sorted and more angular Eocene forearc basin strata document substantial spatial than clasts in the clast-supported conglomerates. variation in sandstone petrofacies. Compared to age- INTERPRETATION O SUCCESSION 1 equivalent sandstones deposited along the southern margin of the forearc basin, sandstone petrofacies from The coarse grain size, sedimentary structures, and the Arkose Ridge Formation are more quartzofeldspathic upward-fining trends of succession 1 indicate that (QFL plot on fig. 5; Winkler, 1978). Sandstones de- streamflow processes were important during deposition. posited along the southern basin margin are interpreted In particular, lithofacies of this succession are to have been derived largely from the Chugach sub- characterized by textures and structures typical of duction complex on the basis of paleocurrent, lithologic, deposits on modern proglacial outwash fans (Boothroyd and compositional data (Little, 1988). Uplift and erosion and Ashley, 1975) and gravelly braided streams (Rust, of the subduction complex provided abundant sedi- 1977). We interpret succession 1 to represent deposition mentary and metasedimentary lithic fragments and a on gravelly bedload streams on an alluvial fan and/or paucity of feldspathic framework grains. In contrast, alluvial braidplain. Organized conglomerate beds at the sediment deposited along the northern basin margin base of each fluvial sequence were likely deposited by received abundant feldspathic detritus from the inactive downstream migration of longitudinal and transverse Mesozoic and active Cenozoic magmatic arcs. gravel bars (Nemec and Steel, 1984). Overlying sandstones are interpreted as being deposited in shallow SEDIMENTOLOGY channels and on emergent bars. Thin mudstone sequences record abandonment of gravelly streams and The Arkose Ridge Formation has been mapped and deposition in adjacent floodplain and paludal swamp described, but no detailed sedimentological data have environments. Rare matrix-rich conglomerates most been previously reported. On the basis of the general likely represent debris flows based on the poor sorting, lithologies and absence of marine megafossils, previous abundant matrix, and absence of internal organization workers interpret the strata as being deposited in and stratification (Johnson, 1965; Middleton and nonmarine environments (Winkler, 1992). Measured Hampton, 1976). stratigraphic sections, lithofacies analysis, maximum particle size data, and paleocurrent analysis from our DESCRIPTION O SUCCESSION 2 study document three lithofacies successions that better Overlying the coarse conglomeratic strata of define the depositional systems of the Arkose Ridge succession 1 is a thick sequence of medium-bedded Formation. Each succession is described and interpreted amalgamated sandstones with interbedded mudstone. below. Individual sandstone bodies are 0.6–2.0 m thick, DESCRIPTION O SUCCESSION 1 lenticular over 10–20 m, and comprised of fine- to medium-grained sandstone (fig. 4). Although individual The lowermost 200 m of the Arkose Ridge Formation sandstone beds are lenticular, beds are highly is characterized by clast-supported cobble and boulder amalgamated forming broad sandstone sheets (figs. conglomerate containing poorly sorted, subrounded to 6D, E). Medium-scale planar, trough, and horizontal rounded clasts (figs. 4, 6C). Average maximum clast cross-stratification are common in the sandstones sizes range from 6 to 113 cm and progressively decrease (fig. 6E). Basal scours have relief of ~0.25–0.80 m and upsection (fig. 4). Clasts as large as 360 cm are present commonly include fossilized tree branches and leaves. locally. Lithofacies assemblages are dominated by Succession 2 is characterized by 2–20 m thick upward- amalgamated, massive to crudely bedded conglomerate fining sequences (fig. 4). Typical sequences consist of 138 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY

Figure 6A. Photomicrograph of lathwork volcanic lithic fragment common in the upper part of the Arkose Ridge Formation. Grain boundary is outlined by white line.

Figure 6B. Photomicrograph of granitic rock fragment representative of igneous rock fragments throughout the Arkose Ridge Formation. Grain comprises plagioclase feldspar (P) and monocrystalline quartz (Q). Grain boundary is outlined by white line.

Figure 6C. Steeply dipping, boulder and cobble con- glomerate deposits of succession 1. White line defines bedding. Note person for scale (arrow, upper right center).

Figure 6D. Sandstone and lava of succession 2. Exposure includes 20-m-thick lava unit that is outlined by arrows and is darker in color than the sandstone beds. Sandstones consist of highly amalgamated, broad, shallow channels. Exposure is approximately 250 m thick. SEDIMENTOLOGY AND PROVENANCE O THE PALEOCENE–EOCENE ARKOSE RIDGE ORMATION, SOUTHERN ALASKA 139

Figure 6E. Amalgamated sandstone channels typical of succession 2. Large black arrows outline the erosional base of one channel. Top of channel is dominated by trough cross-stratification (arrowhead). Bedding dips to left. Person (upper center) for scale.

Figure 6F. Ripple-laminated sandstone with clay drapes. Black arrows point toward a clay drape preserved on a ripple slip face. Note laminated sandstone and mudstone near hammer. Hammer (28 cm) for scale.

massive to planar cross-stratified sandstones that grade braidplain. Evidence for fluvial deposition includes the upward into ripple-laminated, fine-grained sandstones presence of planar and trough cross-stratification, which are capped by structureless to laminated upward-fining packages, and lenticular bed geometries. mudstones with abundant plant fossil leaves. Mudstone The amalgamated sheetlike architecture of channel beds are 0.2–10 m thick, structureless to laminated, and sandstones and the scale of individual sedimentary laterally persistent at the outcrop scale (10–50 m). Thin structures (25–50 cm high planar foresets) suggest that interbeds of carbonaceous shale and coal are also deposition occurred by migration of transverse and present. longitudinal sandbars in low-sinuosity braided streams Volcanic strata occur in the upper part of succession generally less than 150 cm deep (Harms and others, 2 and consist mostly of mafic lava flows (fig. 4). At 1982). The amalgamation of sandstone units is attributed Arkose Ridge, lava flows are 5–20 m thick, laterally to frequent switching of streams across an unconfined continuous for 100–300 m, and interbedded with cross- depositional surface whereas upward-fining cycles likely stratified sandstone and mudstone (figs. 4, 6D). resulted from eventual abandonment of channels. Most Although lava flows are common, pyroclastic volcanic mudstones were probably deposited by suspension rocks were not recognized at any of the three study sites. fallout of fine-grained material during major flood INTERPRETATION O SUCCESSION 2 events. The thickness and lateral continuity of interbedded volcanic rocks suggest that lava deposition Succession 2 is interpreted as being formed by occurred in both channel and interchannel regions of the streamflow and volcanic processes on an alluvial fan or alluvial fan or braidplain. 140 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY

DESCRIPTION O SUCCESSION 3 northern margin of the Cook Inlet–Matanuska Valley forearc basin. Lithofacies, paleocurrent, and composi- Succession 3 consists of channelized thin- to tional data indicate that alluvial fan, fluvial, and medium-bedded sandstone, laminated sandstone and estuarine depositional systems dispersed sediment shale, and massive carbonaceous shale. Large-scale, southwestward, along the trend of the Paleocene– inclined heterolithic strata consisting of laminated Eocene magmatic arc (fig. 7a). Three major lithofacies siltstone and thin-bedded sandstone are preserved in successions in the Arkose Ridge Formation record a succession 3. Clay-draped trough and ripple cross- transgressive sequence from gravel-rich alluvial fan and stratification (fig. 6F), flaser bedding, wavy bedding, fluvial systems below to sandy, braided streams and climbing ripples, and bioturbation are characteristic of tidally influenced, sinuous streams above (fig. 7a). The succession 3. Plant megafossils are very abundant and upsection change in depositional environments is exquisitely preserved, particularly in carbonaceous attributed to a progressive relative rise in sea level during mudstones. the Late Paleocene to Middle Eocene. Rising sea level INTERPRETATION O SUCCESSION 3 prompted northward (arcward) onlap of finer-grained, tidally influenced deposystems across coarser-grained The intricate mixture of channelized sandstone and alluvial fan deposits and lava flows. The transgressive mudstone, in addition to the type, scale, and arrangement event recorded by the lithofacies of the Arkose Ridge of sedimentary structures in succession 3 indicates Formation is coeval with global eustatic sea-level curves deposition in a tidally influenced fluvial system (Clifton, that illustrate first-order transgression during the Late 1982; Eisma, 1998), such as the upper reaches of an Paleocene (60–55 Ma) and sea-level highstand during estuary. Sandstone deposition likely occurred by the Early Eocene (55–50 Ma; Haq and others, 1988). migration of tidal-fluvial point bars in upper estuary Additional extrinsic factors may have contributed to the channels, whereas laterally continuous carbonaceous relative rise in sea-level including variations in tectonic mudstones were deposited in nearby tidal-fluvial subsidence and sediment supply. A modern analog for marshes and tidal flats. Flaser to wavy bedding and clay- our depositional model of the Arkose Ridge Formation draped sand ripples common throughout succession 3 is upper Cook Inlet (fig. 7b). Recent lava flows along the suggest repeated fluctuations in current velocity, a Aleutian arc are located within tens of kilometers of their depositional feature characteristic of tidal regimes eruptive centers and merge basinward with braided (Eisma, 1998). Waxing and waning of current velocity streams, tidally influenced streams, and tidal flats along and stand still of currents during high tide produce clay- the Cook Inlet estuary (Kienle and Nye, 1990; Magoon draped ripples (Reineck and Singh, 1973). Inclined and others, 1976; fig. 7b). heterolithic strata are characteristic of deposits formed by sandy point bars in micro- to mesotidally influenced CONCLUSIONS reaches of rivers (Smith, 1985; Thomas and others, 1987). Flow reversal indicators, such as herringbone Sandstone and conglomerate compositional data cross-stratification, were not observed in the Arkose combined with southwestward-directed paleocurrent Ridge Formation. Almost all tidally influenced rivers, indicators suggest that detritus was derived mainly from however, are ebb dominant (Barwis, 1978) and local source terranes exposed directly north of the Castle numerous workers have demonstrated that the Mountain fault system during deposition of the preservation potential of flow reversal indicators in the Paleocene–Eocene Arkose Ridge Formation. more upstream reaches of tidally influenced rivers is low Compositional data reveal a lower petrofacies rich in (for example, Dorjes and Howard, 1975). The relative metamorphic and granitic detritus and an upper abundance of preserved delicate plant-leaf fossils is petrofacies with higher proportions of volcanic detritus. probably a function of proximity to nearby vegetated Initially, metamorphic-rich detritus was eroded from the terrestrial environments and high sedimentation rates in plutonic roots of an inactive Mesozoic magmatic arc, estuarine environments (Eisma, 1998). whereas erosion along an active Cenozoic magmatic arc provided detritus rich in volcanic lithic fragments during PALEOGEOGRAPHY AND DEPOSITIONAL the later stages of sedimentation. MODEL New sedimentological data demonstrate that alluvial fan, fluvial, and estuarine depositional systems New sedimentological and petrological data indicate deposited the Paleocene–Eocene Arkose Ridge that the Paleocene–Eocene Arkose Ridge Formation was Formation along the arcward margin of the Cook Inlet– deposited proximal to active volcanoes and uplifted Matanuska Valley forearc basin. Progressive upsection plutonic segments of a Mesozoic magmatic arc along the changes in sedimentary lithofacies reveal a transgressive

142 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY depositional sequence from gravel-rich, alluvial fan Talkeetna Mountains quadrangle, northern part of systems to sandy, braided streams to tidally influenced, Anchorage quadrangle, and southwestern corner sinuous streams. An increase in relative sea level during of Healy quadrangle, Alaska: U.S. Geological the Late Paleocene to Middle Eocene resulted in Survey Open-File Report 78–558-A, 62 p., 1 sheet, northward onlap of tidally influenced deposystems scale 1:250,000. toward the basin margin and increased preservation of Dickinson, W.R., 1988, Provenance and sediment fine-grained sediment. Interbedded volcanic flows in the dispersal in relation to paleotectonics and upper part of the formation indicate relative proximity to paleogeography of sedimentary basins, in active volcanic centers during deposition. Kleinspehn, K.L., and Paola, Chris, eds., New Perspectives in Basin Analysis: New York, Springer- ACKNOWLEDGMENTS Verlag, p. 3–25. Dickinson, W.R., 1995, Forearc basins, in Busby, C.J., This research was supported by the Donors of the and Ingersoll, R.V., eds., Tectonics of Sedimentary Petroleum Research Fund, administered by the Basins, Cambridge, Blackwell Science, p. 221–262. American Chemical Society. The Purdue Research Dickinson, W.R., Beard, L.S., Brakenridge, G.R., Foundation; the Geological Society of America; and the Erjavec, J.L., Ferguson, R.C., Inman, K.F., Knepp, American Association of Petroleum Geologists provided R.A., Lindberg, F.A., and Ryberg, P.T., 1983, partial funding. We thank A. Grantz, G. Winkler, and W. Provenance of North American Phanerozoic Nokleberg for useful discussions on the geology of the sandstones in relation to tectonic setting: Geological Talkeetna Mountains and G. Winkler and D. 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Haq, B.U., Hardenbol, Jan, and Vail, P.R., 1988, Geological Survey Journal of Research, v. 2, Mesozoic and Cenozoic chronostratigraphy and p. 323–329. cycles of sea-level change, in Wilgus, C.K., Hastings, Madden-McGuire, D.J, Silberman, M.L., and Church, B.S., Ross, C.A., Posamentier, H.W., Van Wagoner, S.E., 1990, Geologic relationships, K-Ar ages, and John, Ross, C.A., and Kendall, C.G. St. C., Sea-Level isotopic data from the Willow Creek gold mining Changes: An Integrated Approach: Society of district, southern Alaska, in Keays, R.R., Ramsey, Economic Paleontologists and Mineralogists Special W.H.R., and Groves, D.I., eds., Geology of gold Publication No. 42, p. 78–108. deposits—The prospective in 1988: Economic Harms, J.C., Southard, J.B., and Walker, R.G., 1982, Geology, Monograph 6, p. 242–251. Structure and sequences in clastic rocks: Society of Magoon, L.B., Adkison, W.L, and Egbert, R.M., 1976, Economic Paleontologists and Mineralogists, Short Map showing geology, wildcat wells, Tertiary plant Course No. 9, p. 3-1-3-51. fossil localities, K-Ar age dates, and petroleum Heller, P.L., and Dickinson, W.R., 1985, Submarine operations, Cook Inlet area, Alaska: U.S. Geological ramp facies model for delta-fed, sand-rich turbidite Survey Miscellaneous Investigations Map I–1019, systems: American Association of Petroleum 3 sheets, scale 1:250,000. Geologists Bulletin, v. 69, p. 960–976. Marsaglia, K.M., and Ingersoll, R.V., 1992, Ingersoll, R.V., 1979, Evolution of the Late Cretaceous Compositional trends in arc-related, deep-marine forearc basin, northern and central California: sand and sandstone: A reassessment of magmatic-arc Geological Society of America Bulletin, v. 90, provenance: Geological Society of America Bulletin, p. 813–826. v. 104, p. 1637–1649. Ingersoll, R.V., Bullard, T.F., Ford, R.L., Grimm, J.P., Martin, G.C., and Katz, F.J., 1912, Geology and coal Pickle, J.D., and Sares, S.W., 1984, The effect of fields of the lower Matanuska Valley, Alaska: U.S. grain size on detrital modes: A test of the Gazzi- Geological Survey Bulletin 500, 98 p. Dickinson point-counting method: Journal of Miall, A.D., 1978, Lithofacies types and vertical profile Sedimentary Petrology, v. 54, p. 103–116. models in braided river deposits: a summary, in Miall, Johnson, A.M., 1965, A model for debris flow: A.D., ed., Fluvial Sedimentology: Canadian Society University Park, PA, Ph.D. thesis, Pennsylvania State of Petroleum Geologists, Memoir 5, p. 597–604. University. Middleton, G.V., and Hampton, M.A., 1976, Subaqueous Kienle, Juergen, and Nye, C.J., 1990, Volcanoes of Alaska, sediment transport and deposition by sediment in Wood, C.A., and Kienle, Juergen, eds., Volcanoes gravity flows, in Stanley, D.J., and Swift, D.J.P., eds., of North America: Cambridge, MA, p. 8–110. Marine Sediment Transport and Environmental Kirschner, C.E., and Lyon, C.A., 1973, Stratigraphic and Management: New York, Wiley, p. 197–218. tectonic development of Cook Inlet Petroleum Morris, W.R., and Busby-Spera, C.J., 1988, Sedimen- Province, in Pitcher, M.G., ed., Arctic Geology: tologic evolution of a submarine canyon in a forearc American Association of Petroleum Geologists, basin, Upper Cretaceous Rosario Formation, San Memoir 19, p. 396–407. Carlos, Mexico: American Association of Petroleum Kuenzi, W.D., Horst, O.H., and McGehee, R.V., 1979, Geologists Bulletin, v. 72, p. 717–737. Effect of volcanic activity on fluvial-deltaic Morris, W.R., Smith, D.P., and Busby-Spera, C.J., 1989, sedimentation in a modern arc-trench gap, Deep marine conglomerate facies and processes in southwestern Guatemala: Geological Society of Cretaceous forearc basins of Baja California, America Bulletin, v. 90, p. 827–838. Mexico, in Colburn, I.P., Abbott, P.L., and Minch, Little, T.A., 1988, Tertiary tectonics of the Border John, eds., Conglomerates in basin analysis: A Ranges fault system, north-central Chugach symposium dedicated to A.O. Woodford: Society of Mountains, Alaska—Sedimentation, deformation, Economic Paleontologists and Mineralogists, Pacific and uplift along the inboard edge of a subduction Section, v. 62, p. 123–142. complex: Stanford, CA, Stanford University, Ph.D. Nemec, W., and Steel, R.J., 1984, Alluvial and coastal thesis, 565 p. conglomerates: Their significant features and some Little, T. A., 1990, Kinematics of wrench and divergent- comments on gravelly mass-flow deposits, in Koster, wrench deformation along a central part of the E.H., and Steel, R.J., eds., Sedimentology of gravels Border Ranges fault system, northern Chugach and conglomerates: Canadian Society of Petroleum Mountains, Alaska: Tectonics, v. 9, p. 585–611. Geologists, Memoir 10, p. 1–31. MacKevett, E.M., Jr., and Plafker, George, 1974, The Oil and Gas Journal, 1998, Independents lead resurgence Border Ranges fault in south-central Alaska: U.S. of drilling in Alaska’s Cook Inlet, v. 96, p. 27–29. 144 SHORT NOTES ON ALASKA GEOLOGY 1999 TROP AND RIDGWAY

Pavlis, T.L., and Crouse, G.W., 1989, Late Mesozoic Smith, D.G., 1985, Modern analogs of the McMurray strike slip movement on the Border Ranges fault Formation channel deposits, sedimentology of system in the eastern Chugach Mountains, southern mesotidal-influenced meandering point bars with Alaska, in Page, R.A., ed., Special section on inclined beds of alternating mud and sand: Alberta northern Chugach Mountains–southern Copper Oil Sands Technology and Research Authority, River basin segment of the Alaskan Transect; Part I: Final Report for Research Project No. 391, 78 p. Journal of Geophysical Research, B, Solid Earth and Thomas, R.G., Smith, D.G., Wood, J.M., Visser, J., Planets, v. 94, p. 4321–4332. Calverley-Range, E.A., and Koster, E.H., 1987, Plafker, George, Jones, D.L, and Pessagno, E.A., Jr., Inclined heterolithic stratification—terminology, 1977, A Cretaceous accretionary flysch and melange description, interpretation, and significance: terrane along the Gulf of Alaska margin, in Blean, Sedimentary Geology, v. 53, p. 123–179. K.M, The United States Geological Survey in Triplehorn, D.M., Turner, D.L., and Naeser, C.W., 1984, Alaska—Accomplishments during 1976: U.S. Radiometric age of the Chickaloon Formation of Geological Survey Circular 751-B, p. B41–B43. south-central Alaska—Location of the Paleocene– Plafker, George, and Berg, H.C., 1994, Overview of the Eocene boundary: Geological Society of America geology and tectonic evolution of Alaska, in Plafker, Bulletin, v. 95, p. 740–742. George, and Berg, H.G., The Geology of Alaska: The Vessel, R.K., and Davies, D.K., 1981, Nonmarine Geology of North America, v. G–1, p. 989–1021. sedimentation in an active forearc basin: Special Reineck, H.E., and Singh, I.B., 1973, Depositional Publication of the Society of Economic Sedimentary Environments, New York, Springer- Paleontologists and Mineralogists, no. 31, p. 31–45. Verlag, 439 p. Winkler, G.R., 1978, Framework grain mineralogy and Rust, B.R., 1977, Depositional models for braided provenance of sandstones from the Arkose Ridge and alluvium, in Miall, A.D., ed., Fluvial Sedimentology, Chickaloon Formations, Matanuska Valley, in Canadian Society of Petroleum Geologists Memoir Johnson, K.M., ed., The United States Geological 5, p. 605–625. Survey in Alaska - Accomplishments during 1977: Silberman, M.L., and Grantz, Arthur, 1984, Paleogene U.S. Geological Survey Circular 772-B, p. B70–B73. volcanic rocks of the Matanuska Valley area and Winkler, G.R., 1992, Geologic map and summary the displacement history of the Castle Mountain geochronology of the Anchorage 1x3 degree fault, in Coonrad, W.L., and Elliot, R.L., eds., The quadrangle, southern Alaska: U.S. Geological United States Geological Survey in Alaska - Survey Miscellaneous Investigations Series Map Accomplishments during 1981: U.S. Geological I–2283, 1 sheet. Survey Circular 868, p. 82–86. Wolfe, J.A., Hopkins, D.M., and Leopold, E.B., 1966, Silberman, M.L., O’Leary, R.M., Csejtey, Bela, Jr., Tertiary stratigraphy and paleobotany of the Cook Smith, J.G., and Connor, C.L., 1978, Geochemical Inlet region, Alaska: U.S. Geological Survey anomalies and isotopic ages in the Willow Creek Professional Paper 398-A, p. A1–A29. mining district, southwestern Talkeetna Mountains, Alaska: U.S. Geological Survey Open-File Report 78–223, 33 p. LATE DEVONIAN (LATE AMENNIAN) RADIOLARIANS ROM THE CHULITNA TERRANE, SOUTH-CENTRAL ALASKA Mun-Zu Won,1 Robert B. Blodgett,2 Karen H. Clautice,3 and Rainer J. Newberry4

ABSTRACT

A late Famennian radiolarian fauna of generally poor (rarely moderate) preservation was extracted from cherts of the “Do” unit in the Chulitna terrane, south-central Alaska. Because of the generally poor state of preservation, this faunal assemblage exhibits both low diversity and low abundance. It contains 13 genera and 20 species. Three species are new: Palaeoscenidium chulitnaensis, Palaeoscenidium planum, and Popofskyellum? tetralongispina. Among the radiolarian species, Holoeciscus foremanae, the two new species of Palaeoscenidium, and the presence of the species Ceratoikiscum mirum indicate that the Do unit radiolarian fauna is of late Famennian age. The faunal composition of the Chulitna terrane is similar to that of late Famennian radiolarian faunas from the Woodford Formation in the Ouachita Mountains of Oklahoma and Arkansas, as well as the Arbuckle Mountains and Criner Hills of Oklahoma. Outside of North America, the Chulitna fauna is similar to that from the Frankenwald, Bavaria, and from the siliceous boulder fauna of the lower Main valley near Frankfurt/Main, Germany (source area: Frankenwald).

INTRODUCTION

The Chulitna terrane, originally defined by Jones and However, their report did not provide illustrations or others (1980), has had a significant role in the develop- taxonomic descriptions of the radiolarians. ment of accretionary terrane models of Alaska. Both In this paper we present the first illustrations and geologic and paleontologic evidence was marshaled to descriptions of elements of the Famennian radiolarian indicate that some of the rocks originated far to the south fauna from the Chulitna terrane. of their present location. Despite the prominent posi- Late Famennian faunas are fairly well known and tion that fossil biota of this terrane have held as evidence reported (Cheng, 1986; Schmidt-Effing, 1988; Braun, of large-scale displacement, most aspects of fossil fau- 1990; Schwartzapfel and Holdsworth, 1996). However, nas and floras of this terrane remain unknown (Blodgett in these reports, species of some late Famennian radi- and Clautice, 1998). olarian assemblages were selectively described or Chert-rich intervals are common in various posi- misidentified. Therefore, some of those species need to tions in the Chulitna terrane stratigraphy. C.D. Blome be described and/or revised. (in Jones and others, 1980, pl. 2) illustrated Upper Ju- rassic radiolarians from the argillite, chert, and LOCALITY REGISTER sandstone unit of the adjacent West Fork terrane. Radi- olarians were also noted by E.A. Pessagno, Jr. (in Jones Cherts were collected from many localities in the and others, 1980, p. A9) from the argillite, sandstone, Do unit of Chulitna terrane in the summer of 1997 by and chert unit in the Chulitna terrane. Faunal lists were members of the Alaska Division of Geological & Geo- provided for two separate localities in this unit. One physical Surveys field mapping party. Late Devonian locality was identified as being of Callovian to early radiolarians were recovered by Won from 12 localities Tithonian (Late Jurassic) age and the other of late and, from those, four localities (fig. 1) yielded speci- Valanginian (Early Cretaceous) age. The oldest radi- mens that are described in this paper. The initials RB, olarian faunas from the Chulitna terrane are of RN, and HA refer to the collectors, R.B. Blodgett, R.J. Famennian (late Late Devonian) age, and these were Newberry, and E.E. Harris, respectively. All illustrated reported from red cherts within the Do unit of Jones material and types (abbreviation CH) are retained by and others (1980, p. A3–A4; pl. 1). These authors noted Won, Pusan National University, Pusan, Republic of that nine localities in the unit were of Famennian age. Korea.

1Department of Marine Science, Natural Science College, Pusan National University, Pusan, Republic of Korea 609-735. Email for Mun-Zu Won: [email protected] 2Departments of Zoology and Geosciences, Oregon State University, Corvallis, Oregon 97331. 3Alaska Division of Geological & Geophysical Surveys, 794 University Avenue, Suite 200, Fairbanks, Alaska 99709-3645. 4Department of Geology and Geophysics, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, Alaska 99775-5780.

145 146 SHORT NOTES ON ALASKA GEOLOGY 1999 WON AND OTHERS

150°00 149°30’

63°15’ 55420420 63°15’ 2233 2244 1199 2200 2211 2222 2233 2244 1199 2200 2211 2222 2233 2244 CColorado 1199 CCostello o o 0 l 5500 o s 0 0 0 t 0 5 22500 0 r e 4000 5 a 4500 0 33500 5 22500 ll 33000 0 0 d o 5000 4450050 0 o 550000 00 00 6275 S 335005 S W E S T CCreek E 0 r 66000000 00 E e N 44000 S e 0 K R 0 T k 5 R Silver King Mine H CCreek E G 55500 D U A r L O e I PPARK e R 22500 O 00 0 k 5 2266 WWILDERNESS2255 BBOROUGH3030 2299 335005 2288 2277 2266 2255 3300000 3300 2299 2288 2277 2626 2255 3300 0 2299 3090 0 55000000 33500500 I A 4000 2250050 L ITN 0 A 00 S 44000000 N 440000 SU E A- 00 DDENALI SK 445005 U 44500500 N 00 TA 0 5 33000 A 50 33500 00 44500 MMATANUSKA-SUSITNA 44500 0 550000 50 22500 00 0 500 44000 4400000 Creek 5000 0 0

0

0 0 0 0 0 0 0 660000 5 0 44500 55000 Creek 00 66172172 L 445005 Lookout 22965965 445005 0 r A 00 3350050 w Mtn F 66000 MMawra N 00 O R 22000 3355 3366 00 3311 3322 3333 0 3344 3355 3366 3311 3322 3333 3355 3366 3311 0 3322 555005 0 R 0 0 n IO 0 y 0 r K 0 330000 BBryn 0 00 T 55000 500 0 44500 44000 5050 VABM k A Joyce e e 6363 r NNATIONAL CCreek 66000 Golden Zone 0 d 0 0 0 0 00 n i 33500500 Mine 5 l 55500 505000 4000 DITCH BBlind 4000 4000 5500

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t 0 500 0 0 0 33500 s 5 0 22000 o 44500 0 0 0 0 0 0 44000 CCostello 0 0 66000 55000 55000 00 0 3000

555005 5500 44708708 00 H 55500500 C IT 4500 DDITCH 55500 445005 LoLong 50 00 66000 0 0 0 n 0 0 55000 00 50 g 0 55000 55500 0 0 DENALI 44000 0 000 66000 0 0 00 5550050 00 66154154 0 55000 1100 11 1122 7 8 9 1010 11 1122 7 8 11 1212 7 8 9 33500 5 0 445005 0 00

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r 0 55500 e 0 5 33000 0 e 0 0 0 k 44000 0 0 0 0 22000 0 0 0 0 44000 0 0 55000 0 5 555005 22500 00 0 CCopeland 55500500 4500 E o 66000 pe 00 la 4013 0 nd 550000 66000 33500 97RB126 00 33000 0 500 44500500 0 0 G 44500500 0 0 0 5500 0 0 0 55000000 0 0 44000 0 0 22000 850 0 55850 0 2323 2244 1199 2200 2211 2222 N 2233 2244 1199 2200 0 440002211 2222 2233 2244 1199 2200 0 Ohio 55000 5 0 44500 55500 00 000 5 33000 0 33000 0 0 0 40040000 0 000 00 33500 A 44000 335005 0 5 3300000 0 CCreek 0 re 335005 0 44500 4500 e 00 50 5 33500 0 k 335005 22500500 RRANG 0 00 0 0 6600000 0 5 44500 66480480 00 33000 555005 0 2000 303000 k 0 00 Creek 0 0 e 0 5550050 0 0 404000 0 e 50 22000 r 44930930 33500 0 0 1953 CCreek 0 0 50 225005 0 2266 2255 33500 3300 2299 2288 2277 2266 2525 3300 00 2299 2288 2277 2626 2255 3300 2299 0 0 44000 44000 0 0 5 44500 0 n 0 55000 0 0 0 o 0 0 5 y 44500 33500 n 5 33000 a 0 0 0 44000 CCanyon 0 0 0 000 0 0 3500 0 44000 22500 0 3000 5 5 22500 0 45045000 0 330000 49349300 00 44500500 0 0 55000000 0 44000 0

5005000 K

5310 0 0 0 R Creek 5 0 44500 5 33500 0 O 0 0 0 3535 3366 3131 3322 3333 003344 3355 3366 3311 3322 33500 3333 3344 3355 3322 55000 97RN5015 3366 FFORK 44000 3311 0 0 0 0 5 44500 0 0 0 31 32 4500 33000 1900 rk 0 o 4000 0 0 3500 5 0 FFork Chisty 33500 8 22800 T 2020 S T 2121 S 6208 00 33000 335005 440000 33000 0 00 97HA15315 0 0 0 00 0 0 445005 0 0 0 55000000 0 5 5 44500 55500 00 0 550000 0 555005 5 00 2500 0 22500 0 5778 0 0 0 0 2 1 6 5 4 3 33000 2 1 6 5 4 3 2 1 6 0 0 2 5 0 22200 0 0 303000 44000 4450050 44000 550000 0 00 0 0 e 6 l 5 0 00 66000000 33000 d 0 d 0 i 555005 0 0000 00 0 MMiddle 0 44500 LLong 55000 50 0 5275 0 00 o 44000 n

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0 0 8 11800 1166 1155 1144 1133 1188 1177 1166 1155 1144 1133 1188 1717 1177 4000 1144 1133 1188 0 0 0 0 0 5 44000 0 44500 0 5 Creek 2540 33500 0 0 0 22000 0 0 3 5 0 0 55500 0 0 33000 0 0 0 8 5 1529 11800 66000 33500 0 0 0 979 66930930 22979 00 D 440000 A O R 44830830 IL 0 A 0 RRAILROAD 5500 33327327 0 5500 22000 0 00 0 0 5 0 0 0 44500 5 0 66000 0 11500 5 0 22500 0 11500 5559059 5 0 22000 33500 0 5 0 0 2233 2244 1199 2200 2211 0 2222 2233 2244 1199 2200 2211 2222 2233 2244 1199 0 2200 55000000 22500 97RN321 5 1851850 0 0 Mile 290 44500500 5472 55500 00 5 330000 445005 0 00 0 33000 0 0 2966 0 A 0 5 0 22500 K S AY A L 0 W 0 6096 5203 8 AALASKA H 11800 2000 d 0 G 0 I n 0 HHIGHWAY 33500 a 22000 0 5 l 0 ALASKA 0 0 1700 0 5 0 e VABM 11500 4000 0 p Long 44000 22000 R 000 o 1985 r 445005 440000 CCr 2266 2255 3030 00 2299 00 2288 2277 2266 2255 3300 229CCopeland9 2288 2277 2266 E 2255 3300 2299 44000 0 V 1459 0 0 I 55000 1850 0 0 55500500 RRIVER 0 Honolulu 55000000 S 1950 E lu K lu R H no TTHE o PAPARKS 440000 0 0 0 HHonolulu 0 5 33500 0 5 3006 7 330000 11750 445005 00 00 0 0 8 11800

0 5380 0 3344 3535 3366 3311 3322 3355 3366 3311 3322 3333 Shotgun3344 3355 0 3366 3311 3232 3333 0 445005 0 9 00 0 0 225005 11900 5 0 0 11500 22000

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0 0 5 00 44500 0 0 0 33000 0 33000 44000 0 0 0 5 0 0 50 7 0 0 22500 11750 5 A 4000 Little 33500 HHonoluly 5 4 3 2 1 6 5 4 3 N 2 1 on 6 5 2 1 6 T olu ek IITN ly re L 1599 4710 CCreek UUL 0 H 44000000 0 7 CCH 11700 33500 5500000 5 0 0 0 0 un 0 Creek g OOhio 44500500 hot 8 00 SShotgun 11800 115005 h

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0 5 6 11650 0 0 0 0 0 22000 44000 33000 440000 0 1500 000 0 0 5 0 6 5504504 0 11600 BM Hurricane 500 330000 33500 1630 00 Gulch 4680 CCreek 0 4450050 CCoal r 0 e o 0 e 33000 0 a 0 2500 0 0 22000 k 0 l 5 2435 1144 0 1133 1818 1177 1166 1155 22500 1144 1133 1188 1177 1166 1155 1144 1133 1188 1177 1500

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R R 0 2000 R11W R R10W R 5 r 252500 0 R R 00 0 11500 33000 44000 63°00’ 2233 2244 1199 2200 000 2211 2222 2233 2244 1199 2200 2211 2222 2233 2244 1199 2200 63°00’ 150°00’ 149°30’

Figure 1. Index map showing location of fossil localities yielding Famennian radiolarians in the Chulitna terrane, Healy A-6 Quadrangle. Numbered squares are sections, one mile square.

•97RB126: Varicolored chert (green, maroon, gray, •97RN501: Chert beds 1 to 3 cm thick with shaly or light brown) collected from rubble and small partings. These banded cherts underlie massive outcrop on northwest side of small hill atop mesa sandy tuff exposed on the south side of ridge (at an in NE¼ SW¼ NW¼ sec. 21, T. 20 S., R. 11 W., elevation of about 4,700 ft) in NE¼ NW¼ SW¼ Healy A-6 Quadrangle. Lat. 63°10.2' N., Long. sec. 31, T. 20 S., R. 11 W., Healy A-6 Quadrangle. 149°40.6' W. Lat. 63°08.3' N., Long.149°44.4' W. •97RN321: Red and green chert beds exposed atop •97HA1: Massive or brecciated maroon chert and small hill along southeast-trending ridge crest in greenstone exposed on southeast side of hill in NE¼ SW¼ NW¼ SW¼ sec. 22, T. 21 S., R. 12 W., NE¼ NW¼ sec. 1, T. 21 S., R. 12 W., Healy A-6 Healy A-6 Quadrangle. Lat. 63°04.7' N., Long. Quadrangle. Lat. 63°07.9' N., Long. 149°45.7' W. 149°50.4' W. LATE DEVONIAN (LATE AMENNIAN) RADIOLARIANS ROM THE CHULITNA TERRANE, SOUTH-CENTRAL ALASKA 147

BIOSTRATIGRAPHIC RANGE, cause of selective treatment of the faunal elements in CHARACTERISTICS, AND COMPOSITION earlier reports, the species was not previously described. O THE RADIOLARIAN ASSEMBLAGE Specimens of Palaeoscenidium chulitnaensis n. sp. were first reported by Braun (1990) from the Kieselschiefer- The Famennian radiolarian assemblage of the Geröllen fauna, even though he identified this species Chulitna terrane is low both in diversity and abundance, as P. cladophorum. P. chulitnaensis n. sp. does not oc- and many of the specimens have robust skeletons. Late cur in the Woodford Formation but it is present in Famennian radiolarian assemblages having a diverse east-central Alaska together with P. planum n. sp. and species composition are well known from the Woodford H. foremanae. Species of Ceratoikiscum cannot be con- Formation of the Ouachita Mountains of Oklahoma and fidently identified due to poor preservation in the Arkansas (Cheng, 1986) and from the Arbuckle Moun- Chulitna fauna. However, one of them most likely be- tains and Criner Hills of Oklahoma (Schwartzapfel and longs to C. mirum, and another species is more similar Holdsworth, 1996). Late Devonian assemblages of low to specimens of C. sandbergi identified by Schwartzapfel diversity are also known from the Frankenwald and Holdsworth (1996) than to those identified by Cheng (Schmidt-Effing, 1988) and boulders from the lower (1986). C. sandbergi itself was established by Cheng Main valley near Frankfurt/Main in Germany (Braun, (1986) on the basis of poorly preserved specimens and 1990). later redefined by Schwartzapfel and Holdsworth (1996). The Chulitna fauna yields Holoeciscus foremanae However, it is unclear whether these authors were study- Cheng 1986, Entactinia variospina Won 1983, ing the same species because there are several types Plenospongiosa? gourmelonae Won 1998, (species) with well-developed caveal ribs bearing a dis- Archocyrtium venustum Cheng 1986, Archocyrtium tinct rib spine developed near the distal end of each rib, reideli Deflandre 1972, Entactinosphaera? palimbola which is one of the diagnostic characteristics of Foreman 1963, Popofskyellum? tetralongispina n. sp., C. sandbergi. These types (species) show phylogenetic Palaeoscenidium chulitnaensis n. sp., Palaeoscenidium changes with time in unreported faunas from east-cen- planum n. sp., Ceratoikiscum mirum Cheng 1986, tral Alaska. A species of Pylentonema with spines Ceratoikiscum sp. cf. C. sandbergi Cheng 1986, arranged as they are in Quadrapesus Cheng 1986 was Palaeoscenidium sp., Entactinia sp., Popofskyellum? sp., recovered in the Chulitna fauna. The genus Quadrapesus Pylentonema sp., Cyrtentactinia? sp., Totollum? sp., is a synonym of Pylentonema, because, as Won (1998) Tetrentactinia? sp., and Retentactinia? sp. has documented, the different position of the spines can- Comments on the general nature of the radiolarian not be a critical criterion at genus-level classifications. fauna of the Do unit were provided by B.K. Holdsworth, The composition of the fauna discussed in this pa- who reported that the fauna included Entactinosphaera per is similar to that of the Kieselschiefer-Geröllen fauna aitpaiensis Nazarov 1975 and a new species of (Braun, 1990) in boulders from the Rhine River valley, Holoeciscus Foreman 1963, a genus known only from Germany. Braun (1990) concluded that there is no pos- rocks of Famennian age (Jones and others, 1980, p. A4). sibility that his fauna belongs to the Carboniferous there He assigned the fauna a Famennian age. The new because the co-occurring conodont, Bispathodus stabilis, Holoeciscus was later named as Holoeciscus foremanae is known to be limited to Upper Devonian strata. He by Cheng (1986); it was found in the late Famennian assigned the fauna to the Holoeciscus-2 Zone because Woodford Formation. of the presence of Holoeciscus foremanae and absence Among the elements of the Chulitna fauna, species of Pylentonema, citing the report of Cheng (1986), who of Entactinia, Plenospongiosa? gourmelonae, and spe- stated that Pylentonema first occurs at the base of the cies of Archocyrtium are long ranging, extending from Holoeciscus-3 Assemblage Zone. The Chulitna fauna the Late Devonian into the Early Carboniferous. contains Pylentonema. Thus, this fauna is not older than Entactinia variospina has been reported in rocks as old Holoeciscus-3 Assemblage or the base of the as the A. pseudoparadoxa Zone (Won 1991, 1998) of Pylentonema spp.–Staurentactinia spp. Interval Zone Early Carboniferous (Tournaisian) in Europe. It also oc- (Zone I) established by Schwartzapfel and Holdsworth curs in the late Famennian faunas of east-central Alaska. (1996). However, we cannot conclude that the Chulitna From the Chulitna fauna, Palaeoscenidium planum, fauna is younger than the Kieselschiefer-Geröllen fauna Palaeoscenidium chulitnaensis, Holoeciscus foremanae, because of the presence of the genus Pylentonema in and species of Ceratoikiscum and Pylentonema can be Chulitna fauna and its absence in latter fauna. The ab- used for determining the stratigraphic range of the fauna. sence of a species cannot be critical for determining According to observations by Won, one of the newly stratigraphic age, particularly when we consider that established species, Palaeoscenidium planum n. sp., some faunas of any age are not well preserved or are of occurs in the Woodford Formation, Oklahoma, but be- low diversity. 148 SHORT NOTES ON ALASKA GEOLOGY 1999 WON AND OTHERS

The species composition of the Chulitna fauna de- Popofskyellum by the lack of opposed lateral rods. scribed here is also similar to that of Woodford Forma- Later, Schwartzapfel and Holdsworth (1996) estab- tion faunas reported by Cheng (1986) and Schwartzapfel lished a new genus, Totollum, most of whose species and Holdsworth (1996), and of the unreported faunas from were once placed under the emended genus east-central Alaska. Schwartzapfel and Holdsworth Popofskyellum by Cheng. Totollum is defined as (1996) reported that Holoeciscus foremanae occurs in having two subapical horns and two lateral rods, and the Woodford Formation faunas from the Pylentonema the genus Popofskyellum was re-emended by spp.–Staurentactinia spp. Zone (Zone I) to the Schwartzapfel and Holdsworth. In reality, there are Protoalbaillella deflandrei–Holoeciscus spp. Zone species whose characteristics do not match any of (Zone IV). Schwartzapfel and Holdsworth (1996) stated these three genera but are intermediate. Thus, more that the last appearance datum of Ceratoikiscum research is needed to correctly relate these genera. sandbergi is near the top of the Lapidopiscum spp.– Popofskyellum? tetralongispina Won n. sp. Protoalbaillella deflandrei Zone (Zone III). According Figures 2.1-2.3 to Cheng (1986) C. mirum occurs in three strata in the Woodford Formation ranging from the Lapidopiscum Derivation of name: tetra (Lat.), four; longus (Lat.), spp.–Protoalbaillella deflandrei Zone (Zone III) to the long; spina (Lat.), spine middle of the Protoalbaillella deflandrei–Holoeciscus Holotype: CH554 from locality 97RB126 spp. Zone (Zone IV) of Schwartzapfel and Holdsworth Diagnosis: A skeleton consisting of a smooth, bell- (1996). In contrast to this, Schwartzapfel and Holds- shaped to conical shell with hexagonal meshwork; worth (1996) stated that the species occurs from the four thick and long spines around the apical area. Staurentactinia spp.–Lapidopiscum spp. Zone (Zone II) No lateral rods, wings, or rings present. to the Lapidopiscum spp.–Protoalbaillella deflandrei Description: The skeleton is hexagonally latticed, but Zone (Zone III). According to Cheng (1986), the first the apical area from which the four stout spines arise appearance datum of Archeocyrtium venustum and seems to be unperforated. The skeleton is bell-shaped Archeocyrtium riedeli is from the Lapidopiscum spp.– or somewhat conical and smooth, without undula- Protoalbaillella deflandrei Zone (Zone III) established tions or segments. The shape of the hexagonal by Schwartzapfel and Holdsworth (1966). According meshwork can be somewhat compressed in the di- to Won’s observations of Woodford Formation faunas, rection of the long axis, and the mesh is relatively the last appearance datum of the Palaeoscenidium coarse in comparison to that of other species of this planum n. sp. is the same as that of Holoeciscus genus. There are no lateral rods, lateral wings, or foremanae in the Woodford Formation. In unreported rings. One spine is apical, and the other three are late Famennian to early Carboniferous faunas from east- lateral on the apical area. The length and thickness central Alaska, Plenoentactinia? gourmelone occurs in of the spines are similar. faunas equivalent to the Lapidopiscum spp.– Remarks: This species has no lateral rods and so can- Protoalbaillella deflandrei Zone (Zone III) fauna es- not belong to Popofskyellum, according to the tablished by Schwartzapfel and Holdsworth (1996). The emendation by Cheng (1986) or by Schwartzapfel last appearance datum of Entactinosphaera? palimbora and Holdsworth (1996). They all stated that the ge- is the middle of Zone III. Consequently, the stratigraphic nus has no lateral rods. However, this species cannot range of the Chulitna fauna belongs to the Lapidopiscum belong to Kantollum (Cheng, 1986) having no lat- spp.–Protoalbaillella deflandrei Zone (Zone III). Ac- eral rods, because Kantollum, in Cheng’s definition, cording to Schwartzapfel and Holdsworth the is also characterized as being strongly or weakly chronostratigraphic placement of Zone III is uppermost lobulate with four or more lobes with strictures at upper Famennian. joints. In contrast, P. tetralongispina n. sp. has a Taxonomic descriptions in this paper are limited to perfectly smooth shell wall. Some specimens of the new species. Kantollum species, e.g., K. undulatum (Cheng, 1986, pl. 1, figs. 8, 13; Schwartzapfel and Holds- SYSTEMATIC PALEONTOLOGY worth, 1996, pl. 3, figs. 2, 4, 13), have clear traces of lateral rods. Also a species with two apical horns, Family Popofskyellumidae Deflandre 1964 which is one of the most characteristic and diag- Genus Popofskyellum Deflandre 1964 nostic features of Totollum (Schwartzapfel and Remarks: Cheng (1986) established a new genus, Holdsworth, 1996), has no lateral rods [for example, Kantollum, whose test is strongly or weakly lobu- Totollum? sp. cf. T. deflandrei (Cheng): Schwartz- late and has four or more lobes with strictures at apfel and Holdsworth 1996, pl. 5, figs. 3, 4], even joints. He stated that this genus differs from though Totollum is defined as having lateral rods. LATE DEVONIAN (LATE AMENNIAN) RADIOLARIANS ROM THE CHULITNA TERRANE, SOUTH-CENTRAL ALASKA 149

Figure 2. A distance of 6 mm on these SEM photographs is equivalent to the length in µm indicated in parentheses. All specimens are from locality 97RB126 unless otherwise noted. 1-3. Popofskyellum? tetralongispina n. sp. 1. Holotype, CH554 (33.3 µm). 2. CH507 (33.3 µm). 3. Note somewhat varying shapes of the hexagonal meshes of the shell wall, CH511 (33.3 µm) 4. Popofskyellum? sp. CH506 (28.5 µm) 5. Totollum? CH469 (28.5 µm) from locality 97RN321 6. Cyrtentactinia? CH555 (28.5 µm) 7. Ceratoikiscum mirum Cheng 1986 CH451 (33.3 µm) 8 . Ceratoikiscum sp. cf. C. sandbergi Cheng 1986 CH497 (33.3 µm) 9-10. Archocyrtium riedeli Deflandre 1972. 9. CH485 (22.2 µm) from locality 97RN321. 10. CH443 (22.2 µm) 11. Archocyrtium venustum Cheng 1986. CH496 (22.2 µm) 12. Specimen having characteristics between A. venustum Cheng 1986 and A. procerum Cheng 1986 and rudiments of spines on the cephalon, CH444 (22.2 µm) 13, 14. Pylentonema sp. CH445 (22.2 µm), CH460 (33.3 µm) 15, 16. Holoeciscus foremanae Cheng 1986. 15. CH465 (50 µm). 16. CH471 (22.2 µm) from locality 97RN321

Figure 2 150 SHORT NOTES ON ALASKA GEOLOGY 1999 WON AND OTHERS

Figure 3 Figure 4 LATE DEVONIAN (LATE AMENNIAN) RADIOLARIANS ROM THE CHULITNA TERRANE, SOUTH-CENTRAL ALASKA 151

Thus, the absence or presence of the lateral rods Braun (1990). However, the new species differs in might not be a critical characteristic for determin- possessing no tent-like structure and has six spines. ing the generic-level classification. The new species, Because of the lack of a tent-like structure, this spe- P. tetralongispina, is provisionally placed under cies cannot belong to Palaeoscenidium. Also, the Popofskyellum. spinules of this species seem to be connected to spongy tissue which may not be preserved in the Family Palaeoscenidiidae Riedel 1967 Chulitna fauna. Even if the spongy tissue were pre- Genus Palaeoscenidium? Deflandre 1953 served and the spinules are equivalent to subfamily retentactininae established by Won (1997), the struc- Palaeoscenidium chulitnaensis Won n. sp. ture of the spinules differs from that of apophyses Figures 3.10, 3.13 and the spines are connected by a meeting point 1990 Palaeoscenidium cladophorum Deflandre: Braun; unlike that of retentactininae. Thus, this species is pl. 1, figs. 4, 5 provisionally placed on its higher level assignment. Derivation of name: From the Chulitna terrane where Palaeoscenidium planum Won n. sp. this species was recovered Figures 3.6-3.9 Holotype: CH498 from locality 97RB126 Diagnosis: A skeleton consisting of six rod-like robust Derivation of name: Planus (Lat.), flat spines bearing spinules. More spinules on four spines Holotype: W1b-1206 from the Woodford Formation, than the other two spines. Two pairs of three spines Oklahoma connected at a meeting point. Diagnosis: A skeleton characterized by four two-bladed Description: The skeleton consists of six very robust basal spines and four apical spines; one of the two rod-like spines connected by a meeting point. From blades of each spine extended and connected with each ray spinules are raised. The spinules are some- the neighboring spine to form a space that resembles what apophysis-like, but the arrangement of the a tent-like structure; spinules developed on the outer spinules is variable. The spinules arise at one posi- side of the blade or along the edge of the blade; two tion and are radially distributed, or are gathered in of the apical spines longer than the other two. one area or haphazardly along the rays. The number Description: The skeleton consists of four basal and and length of the spinules on each ray are different, four apical spines. Each basal spine is two-bladed. but one spine of each pair has many fewer spinules These blades become narrower distally, and the spine than the other spines. The spinules seem to be con- is pointed at the tip. The blades are widened proxi- nected to spongy tissue, which is not well preserved. mally, and each of them is connected smoothly to Remarks: Specimens of this species from boulders in the adjacent spine. The connected parts form a space the Main valley near Frankfurt/Main (Germany) that is equivalent to the tent-like structure. Each pair were identified as Palaeoscenidium cladophorum by of apical spines consists of a long and a short spine,

Figure 3 (left). A distance of 6 mm on these SEM photographs is equivalent to the length in µm indicated in parentheses. All specimens are from locality 97RB126 unless otherwise noted. 1, 2. Retentactinia? sp. Arrows indicate apophyses. 1. CH480 (44.4 µm). 2. CH466 (40 µm) 3. Entactinia sp. CH431(33.3 µm) 4, 5. Tetrentactinia? sp. 4. CH433 (33.3 µm) from locality 97RN321A. 5. CH508 (33.3 µm) 6-9. Palaeoscenidium planum n. sp. 6, 7. CH452 (40, 16.6 µm) 8. W2b-1223 (27.7 µm) from the Woodford Formation, Oklahoma. 9. Holotype: W1b-1206 (27.7 µm) from the Woodford Formation, Oklahoma; one short spine of left pair of apical spines is not visible 10, 13. Palaeoscenidium chulitnaensis n. sp. 10, 13. Holotype: CH498 (40, 16.3 µm) from locality 97RN321 11, 12. Palaeoscenidium sp. 11. CH448 (50 µm) from locality 97HA1. 12. CH500 (40 µm) Figure 4 (left). A distance of 6 mm on these SEM photographs is equivalent to the length in µm indicated in parentheses. All specimens are from locality 97RB126. 1-6. Entactinia variospina Won 1983. 1. CH534 (50 µm). 2. CH423 (50 µm). 3. CH428 (66.6 µm). 4. CH537 (50 µm). 5. CH426 (50 µm). 6. CH425 (66.6 µm) 7, 8. Plenospongiosa? gourmelonae Won 1998. 7. CH436 (40 µm). 8. CH437 (40 µm) 9-18. Entactinosphaera? palimbola Foreman 1963. 9. CH563 (66.6 µm). 10. CH478 (66.6 µm). 11. CH461 (66.6 µm). 12. CH475 (66.6 µm). 13. CH420 (66.6 µm). 14. CH417 (66.6 µm). 15. CH565 (66.6 µm). 16. CH562 (66.6 µm). 17. CH477 (66.6 µm). 18. CH459 (66.6 µm) 152 SHORT NOTES ON ALASKA GEOLOGY 1999 WON AND OTHERS

and they are separated by a very short distance. In Sciences, v. 259, no. 18, p. 3055–3058. each pair, the long spine stands at right angles to the Deflandre, Georges, 1972, Remarques complémentaires short spine. The two longer spines stand at opposite sur la morphologie et la nomenclature de quelques sides, and the two shorter spines stand at opposite genres de Radiolaires du Paléozoïque: Paris, sides. Spinules arise from the outer surface of the Comptes Rendus Hebdomadaires des Seances de blade, at the edges of the blade, or at the edge where l’Académie des Sciences, Serie D: Sciences the two blades meet. Naturelles, v. 275, no. 1, p. 13–16. Remarks: None of the specimens of P. planum n. sp. Foreman, H.P., 1963, Upper Devonian Radiolaria from from the Chulitna terrane is perfectly preserved. the Huron Member of the Ohio Shale: Micro- Thus, some features cannot be observed in the fig- paleontology, v. 9, no. 3, p. 267–304. ure illustrating the specimen from the Chulitna Gourmelon, Francoise, 1987, Les Radiolaires tour- terrane. The description of this species is also based naisiens des nodules phosphatés de la Montagne on well-preserved specimens, one of which is taken Noire et des Pyrénées Centrales: Brest, France, as the holotype, in the Woodford Formation of Okla- Biostratigraphie du Paléozoique, v. 6, p. 1–172. homa. However, Cheng (1986) and Schwartzapfel Jones, D.L., Silberling, N.J., Csejtey, Bela, Jr., Nelson, and Holdsworth (1996) did not describe or illustrate W.H., and Blome, C.D., 1980, Age and structural this species. This species is rare in the Do unit but significance of ophiolite and adjoining rocks in the according to my observation of Woodford Forma- upper Chulitna district, south-central Alaska: U.S. tion faunas, it generally occurs with Holoeciscus Geological Survey Professional Paper 1121-A, 21 p. foremanae in that formation. This new species is Nazarov, B.B., 1975, Radiolyarii nizhnego-srednego clearly distinguished from the other species of paleozoya kazakhstana: Trudy Geologicheskie Palaeoscenidium by the presence of the two-bladed Institut, Akademiya Nauk SSSR, Vypusk 27, 202 p. basal spines. Riedel, W.R., 1967, Some new families of Radiolaria: Proceedings of the Geological Society of London, ACKNOWLEDGMENTS no. 1640, p. 148–149. Schmidt-Effing, Reinhard, 1988, Eine Radiolarien-Fauna We are deeply grateful to Katherine Reed and Paula des Famenne (Ober-Devon) aus dem Franken-wald Noble, both of whom provided technical and prelimi- (Bayern): Geologica et Palaeontologica, v. 22, p. 33–41. nary editorial reviews as well as valuable suggestions Schwartzapfel, J.A., and Holdsworth, B.K., 1996, Upper for additions to the manuscript. The siliceous micro- Devonian and Mississippian Radiolarian Zonation fossil study was supported by Pusan National University, and Biostratigraphy of the Woodford, Sycamore, Korea. Caney and Goddard Formations, Oklahoma: Cushman REERENCES Foundation for Foraminiferal Research; Special Publication 33, 275 p. Won, M.-Z., 1983, Radiolarien aus dem Unter-Karbon Blodgett, R.B., and Clautice, K.H., 1998, New insights des Rheinischen Schiefergebirges (Deutschland): into the stratigraphy and paleontology of the Chulitna Palaeontographica, Abteilung A, v. 182, p. 116–175. terrane and surrounding area, Healy A-6 Quadrangle, ———1990, Lower Carboniferous radiolarian fauna south-central Alaska [abst.]: The Alaska Geological from Riescheid, Germany: Journal of the Paleon- Society, 1998, Science and Technology Conference, tological Society of Korea, v. 6, no. 2, p. 111–146. “Cutting Edge in Alaska,” 2 p. ———1991, Phylogenetic study of some species of Braun, Andreas, 1990, Overdevonische Radiolarien aus genus Albaillella Deflandre 1952 and a radiolarian Kieselschiefer-Geröllen des unteren Maintales bei zonation in the Rheinische Schiefergebirge, West Frankfurt am Main: Geologisches Jahrbuch Hessen, Germany: Journal of the Paleontological Society of v. 118, p. 5–27. Korea, v. 7, no. 1, p. 13–25. Cheng, Y.-N., 1986, Taxonomic Studies on Upper ———1997, Review of the family Entactiniidae Paleozoic Radiolaria: Taiwan, National Museum of (Radiolaria) and taxonomy and morphology of the Natural Science, Special Publication 1, 311 p. Entactiniidae in the late Devonian (Frasnian) Gogo Deflandre, Georges, 1953, Radiolaires fossiles, in formation, Australia: Micropaleontology, v. 43, Grassé, P.P., ed., Traité de Zoologie, Paris, Masson no. 4, p. 333–369. & Cie., v. 1, no. 2, p. 389–436. ———1998, A Tournaisian (Lower Carboniferous) ———1964, La famille des Popofskyellidae fam. nov. radiolarian zonation and radiolarians of the et le genre Popofskyellum Defl., Radiolaires viséens A. pseudoparadoxa Zone from Oese (Rheinische de la Montagne Noire: Paris, Comptes Rendus Schiefergebirge), Germany: The Journal of the Korean Hebdomadaires des Seances de l’Académie des Earth Science Society, v. 19, no. 2, p. 216–259. SHORT NOTES ON ALASKA GEOLOGY 1999 PREVIOUS EDITIONS OF SHORT NOTES ON ALASKA GEOLOGY

Short Notes on Alaskan Geology 1976: DGGS Geologic Report 51 (out of stock, $3.50 photocopied) Reconnaissance geology along the Variegated Glacier, Saint Elias Mountains Evidence for early Cenozoic orogeny in central Alaska Range The Shumagin-Kodiak batholith: A Paleocene magmatic arc? Speculative tectonic evolution of the Cenozoic Shelikof Trough, south-central Alaska Discovery of blueschists on Kodiak Island Large kaolinite crystals in the Chignik Formation (Upper Cretaceous), Herendeen Bay Occurrence of sodic amphibole-bearing rocks in the Valdez C-2 Quadrangle High-quality coal near Point Hope, northwestern Alaska

Short Notes on Alaskan Geology 1977: DGGS Geologic Report 55 (out of stock, $5.00 photocopied) A Givetian (Late Middle Devonian) fauna from Healy B-4 Quadrangle, central Alaska Range Probable karst topography near Jade Mountains, southwestern Brooks Range Tectonic signifi cance of the Knik River schist terrane, south-central Alaska Geochronology of southern Prince of Wales Island Katmai caldera: Glacier growth, lake rise, and geothermal activity Geology and K-Ar age of mineralized intrusive rocks from the Chulitna mining district, central Alaska The Richardson lineament: A structural control for gold deposits in the Richardson mining district, interior Alaska Boulder Creek tin lode deposits Comparison of mercury-antimony-tungsten mineralization of Alaska with strata-bound cinnabar-stibnite-scheelite deposits of the Circum-Pacifi c and Mediterranean regions Earthquake recurrence and location in the western Gulf of Alaska

Short Notes on Alaskan Geology 1978: DGGS Geologic Report 61 ($2.00) Holocene displacements measured by trenching the Castle Mountain fault near Houston Bluff Point landslide, a massive ancient rock failure near Homer Recurrent late Quaternary faulting near Healy Glaciation of Indian Mountain, west-central Alaska The Cantwell ash bed, a Holocene tephra in the central Alaska Range Geochronology of metamorphic and igneous rocks in the Kantishna Hills, Mount McKinley Quadrangle The Chilikadrotna Greenstone, an Upper Silurian metavolcanic sequence in the central Lake Clark Quadrangle Tectonic and economic signifi cance of Late Devonian and late Proterozoic U-Pb zircon ages from the Brooks Range

Short Notes on Alaskan Geology 1979-80: DGGS Geologic Report 63 ($1.00) Lead isotope ratios from the Red Dog and Drenchwater Creek lead-zinc deposits, De Long Mountains, Brooks Range 40K-40Ar ages from rhyolite of Sugar Loaf Mountain, central Alaska Range: Implications for offset along the Hines Creek strand of the Denali fault system Multiple glaciation in the Beaver Mountains, western interior Alaska Fossil algae in Lower Devonian limestones, east-central Alaska Tertiary tillites(?) on the northeast fl ank of Granite Mountain, central Alaska Range Evidence for suprapermafrost ground-water blockage, Prudhoe Bay oil fi eld

Short Notes on Alaskan Geology 1981: DGGS Geologic Report 73 ($3.00) Alkaline igneous rocks in the eastern Alaska Range Shear moduli and sampling ratios for the Bootlegger Cove Formation as determined by resonant-column testing Clinoptilolite and mordenite deposits of possible economic value at Iliamna Lake, Alaska The Keete Inlet thrust fault, Prince of Wales Island Two Holocene maars in the central Alaska Range Radiometric-age determinations from Kiska Island, Aleutian Islands, Alaska Geochemical signature of the Goon Dip Greenstone on Chicagof Island, southeastern Alaska Uranium mineralization in the Nenana Coal Field, Alaska Reconnaissance of rare-metal occurrences associated with the Old Crow batholith, eastern Alaska - north-western Canada A recent earthquake on the Denali fault in the southeast Alaska Range Triassic paleomagnetic data and paleolatitudes for Wangellia, Alaska

153 Short Notes on Alaskan Geology 1982-83: DGGS Professional Report 86 ($2.50) An unconformity with associated conglomeratic sediments in the Berners Bay area of southeast Alaska An iron-rich lava fl ow from the Nenana coal fi eld, central Alaska Results of shallow seismic survey for ground water at McGrath Evaluation of a shallow sand-and-gravel aquifer at Eagle River Correlation of geophysical well logs for a water development in south Anchorage Garnet compositional estimates as indicators of progressive regional metamorphism in polymetamorphic rocks, Kantishna Hills Geology of the Miss Molly molybdenum prospect, Tyonek C-6 Quadrangle Glacial geology of the Mt. Prindle area, Yukon-Tanana Upland

Short Notes on Alaskan Geology 1991: DGGS Professional Report 111 ($8.00) Tin placers associated with the downcutting of fi ssure basalts, Ray River drainage, Alaska Geology and geochemistry of the Gagaryah barite deposit, western Alaska Range, Alaska Geology and geochemistry of Tatlawiksuk Hot Springs, a newly discovered geothermal area in western Alaska Geology and geochemistry of the Sleitat Mountain tin deposit, southwestern Alaska Native mercurian-silver, silver, and gold nuggets from Hunter Creek, Alaska Late Pleistocene volcanic deposits near the Valley of Ten Thousand Smokes, Katmai National Park, Alaska Deglaciation of the Allison-Sawmill Creeks area, southern shore of Port Valdez, Alaska Dating Holocene moraines of Canwell Glacier, Delta River valley, central Alaska Range Gilead sandstone, northeastern Brooks Range, Alaska: An Albian to Cenomanian marine clastic succession Kikiktat Mountain klippe: A link between the Copter Peak and Nuka Ridge allochthons, northcentral Brooks Range, Alaska Sample media useful for a systematic geochemical survey of upper Valdez Creek, Alaska

Short Notes on Alaskan Geology 1993: DGGS Professional Report 113 ($6.00) Mississippian terrigenous clastic and volcaniclastic rocks of the Ellesmerian sequence, upper Sheenjek River area, eastern Brooks Range, Alaska The penultimate great earthquake in southcentral Alaska: evidence from a buried forest near Girdwood Geology, alteration, and mineralization of the Vinasale Mountain gold deposit, west-central Alaska Fumarolic gas chemistry (1982) and thermal spring water chemistry (1985), Crater Peak, Mount Spurr, Alaska Organic-rich shale and bentonite in the Arctic Creek unit, Arctic National Wildlife Refuge: implications for stratigraphic and structural in- terpretations Dating Holocene moraines of Black Rapids Glacier, Delta River valley, central Alaska Range Paleomagnetism of the Fairbanks basalts, interior Alaska The Hayes Glacier fault, southern Alaska Range: evidence for post-Paleocene movement Detachment folds and a passive-roof duplex: examples from the northeastern Brooks Range, Alaska

Short Notes on Alaskan Geology 1995: DGGS Professional Report 117 ($15.00) Radiocarbon age of probable Hayes tephra, Kenai Peninsula, Alaska Geochemistry of saline lakes of the northeastern Yukon Flats, eastcentral Alaska Geometry and deformation of a duplex and its roof layer: Observations from the Echooka anticlinorium, northeastern Brooks Range, Alaska Late-Wisconsin events in the Upper Cook Inlet region, southcentral Alaska Stratigraphy and implications of a lakeside section, Glacial Lake, southwestern Kigluaik Mountains, Seward Peninsula, Alaska Lithofacies, petrology, and petrophysics of the Kemik Sandstone (Lower Cretaceous), eastern Arctic Slope, Alaska A new species of the conodont amydrotaxis From the Early Devonian of southwestern Alaska Early Devonian and Late Triassic conodonts from Annette and Hotspur Islands, southeastern Alaska Mineralization and zoning of polymetallic veins in the Beaver Mountains volcano-plutonic complex, Iditarod Quadrangle, westcentral Alaska Possible thrust windows on the central Seward Peninsula, Alaska

Short Notes on Alaskan Geology 1997: DGGS Professional Report 118 ($9.00) Geochronologic investigations of magmatism and metamorphism within the Kigluaik Mountains gneiss dome, Seward Peninsula, Alaska Composite standard correlation of the Mississippian-Pennsylvanian (Carboniferous) Lisburne Group from Prudhoe Bay to the eastern Arctic National Wildlife Refuge, North Slope, Alaska Enigmatic source of oil from Chukchi Sea, northwestern Alaska Emsian (late Early Devonian) fossils indicate a Siberian origin for the Farewell Terrane Growth-position petrifi ed trees overlying thick Nanushuk Group coal, Lili Creek, Lookout Ridge Quadrangle, North Slope, Alaska Paleotopographic control on deposition of the lower Kayak Shale, northern Franklin Mountains, Brroks Range, Alaska Cooling history of the Okpilak batholith, northeastern Brooks Range, as determined from potassium-feldspar thermochronometry First occurrence of a hadrosaur (Dinosauria) from the Matanuska Formation (Turonian) in the Talkeetna Mountains of south-central Alaska Petrography of the Tingmerkpuk Sandstone (Neocomian), northwestern Brroks Range, Alaska: A preliminary study Lower to Middle Devonian (latest Emsian to earliest Eifelian) conodonts from teh Alexander Terrane, southeastern Alaska Preliminary petrography and provenance of six Lower Cretaceous sandstones, northwestern Brooks Range, Alaska

154 View eastward along the west fork of Atigun River of prominent anticlinal hinge in the Upper Kanayut Conglomerate (Lower Mississip- pian to Upper Devonian) within the Toyuk thrust zone, a major thrust zone in the central Brooks Range. See Chmielowski and Wallace (this volume), “Duplex structure and Paleocene displacement of the Toyuk thrust zone near the Dalton Highway, north-central Brooks Range.” (Photo by Reia M. Chmielowski)

Excavator and bulldozer trench on Lewis Ridge, Donlin Creek property, south- western Alaska. Trench is approximately 1,300 feet long and part of a 3.7-mile trenching program by Placer Dome Exploration Inc. during the 1997 and 1998 fi eld seasons. Light-colored rocks are igneous dikes and sills, and darker rocks are Kuskokwim Group shale and graywacke. Note the core drill rig in the back- ground to the right of center. (Photo by David Szumigala)